diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index e3bdc6daa..1f64701c4 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -1087,10 +1087,14 @@ function(_ep_set_directories name)
   endforeach()
   get_property(source_subdir TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR)
   if(NOT source_subdir)
-    set_property(TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR ".")
+    set_property(TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR "")
   elseif(IS_ABSOLUTE "${source_subdir}")
     message(FATAL_ERROR
       "External project ${name} has non-relative SOURCE_SUBDIR!")
+  else()
+    # Prefix with a slash so that when appended to the source directory, it
+    # behaves as expected.
+    set_property(TARGET ${name} PROPERTY _EP_SOURCE_SUBDIR "/${source_subdir}")
   endif()
   if(build_in_source)
     get_property(source_dir TARGET ${name} PROPERTY _EP_SOURCE_DIR)
@@ -2257,11 +2261,7 @@ function(_ep_add_configure_command name)
       endif()
     endif()
 
-    if(source_subdir STREQUAL ".")
-      list(APPEND cmd "${source_dir}")
-    else()
-      list(APPEND cmd "${source_dir}/${source_subdir}")
-    endif()
+    list(APPEND cmd "${source_dir}${source_subdir}")
   endif()
 
   # If anything about the configure command changes, (command itself, cmake