diff --git a/Modules/ExternalData.cmake b/Modules/ExternalData.cmake
index ee206935e..73a4990f3 100644
--- a/Modules/ExternalData.cmake
+++ b/Modules/ExternalData.cmake
@@ -421,6 +421,7 @@ function(_ExternalData_arg target arg options var_file)
   set(external "") # Entries external to the source tree.
   set(internal "") # Entries internal to the source tree.
   set(have_original ${data_is_directory})
+  set(have_original_as_dir 0)
 
   # Process options.
   set(series_option "")
@@ -470,11 +471,18 @@ function(_ExternalData_arg target arg options var_file)
   endif()
 
   if(NOT have_original)
-    message(FATAL_ERROR "Data file referenced by argument\n"
+    if(have_original_as_dir)
+      set(msg_kind FATAL_ERROR)
+      set(msg "that is directory instead of a file!")
+    else()
+      set(msg_kind AUTHOR_WARNING)
+      set(msg "that does not exist as a file (with or without an extension)!")
+    endif()
+    message(${msg_kind} "Data file referenced by argument\n"
       "  ${arg}\n"
       "corresponds to source tree path\n"
       "  ${reldata}\n"
-      "that does not exist as a file (with or without an extension)!")
+      "${msg}")
   endif()
 
   if(external)
@@ -591,27 +599,33 @@ function(_ExternalData_arg_find_files pattern regex)
       set(alg "")
     endif()
     if("x${relname}" MATCHES "^x${regex}$" # matches
-        AND NOT IS_DIRECTORY "${top_src}/${entry}" # not a directory
         AND NOT "x${relname}" MATCHES "(^x|/)\\.ExternalData_" # not staged obj
         )
-      set(name "${top_src}/${relname}")
-      set(file "${top_bin}/${relname}")
-      if(alg)
-        list(APPEND external "${file}|${name}|${alg}")
-      elseif(ExternalData_LINK_CONTENT)
-        _ExternalData_link_content("${name}" alg)
-        list(APPEND external "${file}|${name}|${alg}")
-      elseif(NOT top_same)
-        list(APPEND internal "${file}|${name}")
-      endif()
-      if("${relname}" STREQUAL "${reldata}")
-        set(have_original 1)
+      if(IS_DIRECTORY "${top_src}/${entry}")
+        if("${relname}" STREQUAL "${reldata}")
+          set(have_original_as_dir 1)
+        endif()
+      else()
+        set(name "${top_src}/${relname}")
+        set(file "${top_bin}/${relname}")
+        if(alg)
+          list(APPEND external "${file}|${name}|${alg}")
+        elseif(ExternalData_LINK_CONTENT)
+          _ExternalData_link_content("${name}" alg)
+          list(APPEND external "${file}|${name}|${alg}")
+        elseif(NOT top_same)
+          list(APPEND internal "${file}|${name}")
+        endif()
+        if("${relname}" STREQUAL "${reldata}")
+          set(have_original 1)
+        endif()
       endif()
     endif()
   endforeach()
   set(external "${external}" PARENT_SCOPE)
   set(internal "${internal}" PARENT_SCOPE)
   set(have_original "${have_original}" PARENT_SCOPE)
+  set(have_original_as_dir "${have_original_as_dir}" PARENT_SCOPE)
 endfunction()
 
 #-----------------------------------------------------------------------------
diff --git a/Tests/Module/ExternalData/CMakeLists.txt b/Tests/Module/ExternalData/CMakeLists.txt
index 5a6f3d50f..ebca48e44 100644
--- a/Tests/Module/ExternalData/CMakeLists.txt
+++ b/Tests/Module/ExternalData/CMakeLists.txt
@@ -23,6 +23,8 @@ ExternalData_Add_Test(Data1
   COMMAND ${CMAKE_COMMAND}
     -D Data=DATA{Data.dat}
     ${Data1CheckSpaces}
+    -D DataMissing=DATA{DataMissing.dat}
+    -D DataMissingWithAssociated=DATA{DataMissing.dat,Data.dat}
     -D SeriesA=DATA{SeriesA.dat,:}
     -D SeriesB=DATA{SeriesB.dat,:}
     -D SeriesC=DATA{SeriesC.dat,:}
diff --git a/Tests/Module/ExternalData/Data1Check.cmake b/Tests/Module/ExternalData/Data1Check.cmake
index 57702453e..485b5c6c2 100644
--- a/Tests/Module/ExternalData/Data1Check.cmake
+++ b/Tests/Module/ExternalData/Data1Check.cmake
@@ -8,6 +8,28 @@ if(DEFINED DataSpace)
     message(SEND_ERROR "Input file:\n  ${DataSpace}\ndoes not have expected content, but [[${lines}]]")
   endif()
 endif()
+if(DataMissing)
+  if(EXISTS "${DataMissing}")
+    message(SEND_ERROR
+      "Input file:\n"
+      "  ${DataMissing}\n"
+      "exists but should not."
+      )
+  endif()
+else()
+  message(SEND_ERROR "DataMissing is not set!")
+endif()
+if(DataMissingWithAssociated)
+  if(EXISTS "${DataMissingWithAssociated}")
+    message(SEND_ERROR
+      "Input file:\n"
+      "  ${DataMissingWithAssociated}\n"
+      "exists but should not."
+      )
+  endif()
+else()
+  message(SEND_ERROR "DataMissingWithAssociated is not set!")
+endif()
 set(SeriesAn1 "1\\.dat")
 set(SeriesBn1 "_1\\.dat")
 set(SeriesCn1 "\\.1\\.dat")
diff --git a/Tests/RunCMake/ExternalData/Directory1-stderr.txt b/Tests/RunCMake/ExternalData/Directory1-stderr.txt
index 85c250f02..2bc3c601d 100644
--- a/Tests/RunCMake/ExternalData/Directory1-stderr.txt
+++ b/Tests/RunCMake/ExternalData/Directory1-stderr.txt
@@ -7,7 +7,7 @@ CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
 
     Directory1
 
-  that does not exist as a file \(with or without an extension\)!
+  that is directory instead of a file!
 Call Stack \(most recent call first\):
   .*
   Directory1.cmake:3 \(ExternalData_Add_Test\)
diff --git a/Tests/RunCMake/ExternalData/Directory3-result.txt b/Tests/RunCMake/ExternalData/Directory3-result.txt
deleted file mode 100644
index d00491fd7..000000000
--- a/Tests/RunCMake/ExternalData/Directory3-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/ExternalData/Directory3-stderr.txt b/Tests/RunCMake/ExternalData/Directory3-stderr.txt
index 56a341e03..ceed2a04d 100644
--- a/Tests/RunCMake/ExternalData/Directory3-stderr.txt
+++ b/Tests/RunCMake/ExternalData/Directory3-stderr.txt
@@ -1,4 +1,4 @@
-CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
+CMake Warning \(dev\) at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
   Data file referenced by argument
 
     DATA{Directory3/\*}
@@ -12,3 +12,4 @@ Call Stack \(most recent call first\):
   .*
   Directory3.cmake:3 \(ExternalData_Add_Test\)
   CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/ExternalData/MissingData-result.txt b/Tests/RunCMake/ExternalData/MissingData-result.txt
deleted file mode 100644
index d00491fd7..000000000
--- a/Tests/RunCMake/ExternalData/MissingData-result.txt
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/Tests/RunCMake/ExternalData/MissingData-stderr.txt b/Tests/RunCMake/ExternalData/MissingData-stderr.txt
index e794f955d..39ed2f15f 100644
--- a/Tests/RunCMake/ExternalData/MissingData-stderr.txt
+++ b/Tests/RunCMake/ExternalData/MissingData-stderr.txt
@@ -1,4 +1,4 @@
-CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
+CMake Warning \(dev\) at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
   Data file referenced by argument
 
     DATA{MissingData.txt}
@@ -10,5 +10,6 @@ CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
   that does not exist as a file \(with or without an extension\)!
 Call Stack \(most recent call first\):
   .*
-  MissingData.cmake:2 \(ExternalData_Add_Test\)
+  MissingData.cmake:4 \(ExternalData_Expand_Arguments\)
   CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/ExternalData/MissingData-stdout.txt b/Tests/RunCMake/ExternalData/MissingData-stdout.txt
new file mode 100644
index 000000000..addd40eaa
--- /dev/null
+++ b/Tests/RunCMake/ExternalData/MissingData-stdout.txt
@@ -0,0 +1 @@
+-- Missing data reference correctly transformed!
diff --git a/Tests/RunCMake/ExternalData/MissingData.cmake b/Tests/RunCMake/ExternalData/MissingData.cmake
index b3c8a5cdc..f5fefd5cc 100644
--- a/Tests/RunCMake/ExternalData/MissingData.cmake
+++ b/Tests/RunCMake/ExternalData/MissingData.cmake
@@ -1,5 +1,10 @@
 include(ExternalData)
-ExternalData_Add_Test(Data
-  NAME Test
-  COMMAND ${CMAKE_COMMAND} -E echo DATA{MissingData.txt}
-  )
+
+set(output "${CMAKE_SOURCE_DIR}/MissingData.txt")
+ExternalData_Expand_Arguments(Data args DATA{MissingData.txt})
+if("x${args}" STREQUAL "x${output}")
+  message(STATUS "Missing data reference correctly transformed!")
+else()
+  message(FATAL_ERROR "Missing data reference transformed to:\n  ${args}\n"
+    "but we expected:\n  ${output}")
+endif()
diff --git a/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stderr.txt b/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stderr.txt
new file mode 100644
index 000000000..315af5e49
--- /dev/null
+++ b/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stderr.txt
@@ -0,0 +1,15 @@
+CMake Warning \(dev\) at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
+  Data file referenced by argument
+
+    DATA{MissingData.txt,Data.txt}
+
+  corresponds to source tree path
+
+    MissingData.txt
+
+  that does not exist as a file \(with or without an extension\)!
+Call Stack \(most recent call first\):
+  .*
+  MissingDataWithAssociated.cmake:4 \(ExternalData_Expand_Arguments\)
+  CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stdout.txt b/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stdout.txt
new file mode 100644
index 000000000..addd40eaa
--- /dev/null
+++ b/Tests/RunCMake/ExternalData/MissingDataWithAssociated-stdout.txt
@@ -0,0 +1 @@
+-- Missing data reference correctly transformed!
diff --git a/Tests/RunCMake/ExternalData/MissingDataWithAssociated.cmake b/Tests/RunCMake/ExternalData/MissingDataWithAssociated.cmake
new file mode 100644
index 000000000..a4c4638b0
--- /dev/null
+++ b/Tests/RunCMake/ExternalData/MissingDataWithAssociated.cmake
@@ -0,0 +1,10 @@
+include(ExternalData)
+
+set(output "${CMAKE_BINARY_DIR}/MissingData.txt")
+ExternalData_Expand_Arguments(Data args DATA{MissingData.txt,Data.txt})
+if("x${args}" STREQUAL "x${output}")
+  message(STATUS "Missing data reference correctly transformed!")
+else()
+  message(FATAL_ERROR "Missing data reference transformed to:\n  ${args}\n"
+    "but we expected:\n  ${output}")
+endif()
diff --git a/Tests/RunCMake/ExternalData/RunCMakeTest.cmake b/Tests/RunCMake/ExternalData/RunCMakeTest.cmake
index 93ff08f59..04e3d593f 100644
--- a/Tests/RunCMake/ExternalData/RunCMakeTest.cmake
+++ b/Tests/RunCMake/ExternalData/RunCMakeTest.cmake
@@ -15,6 +15,7 @@ run_cmake(LinkContentMD5)
 run_cmake(LinkContentSHA1)
 run_cmake(LinkDirectory1)
 run_cmake(MissingData)
+run_cmake(MissingDataWithAssociated)
 run_cmake(NoLinkInSource)
 run_cmake(NoURLTemplates)
 run_cmake(NormalData1)