diff --git a/Base64.c b/Base64.c
index d07bdd01b..4b8ede2fb 100644
--- a/Base64.c
+++ b/Base64.c
@@ -115,10 +115,10 @@ void kwsysBase64_Encode1(const unsigned char *src, unsigned char *dest)
    actually knowing how much data to expect (if the input is not a multiple of
    3 bytes then the extra padding needed to complete the encode 4 bytes will
    stop the decoding anyway).  */
-unsigned long kwsysBase64_Encode(const unsigned char *input,
-                                 unsigned long length,
-                                 unsigned char *output,
-                                 int mark_end)
+size_t kwsysBase64_Encode(const unsigned char *input,
+                          size_t length,
+                          unsigned char *output,
+                          int mark_end)
 {
   const unsigned char *ptr = input;
   const unsigned char *end = input + length;
@@ -157,7 +157,7 @@ unsigned long kwsysBase64_Encode(const unsigned char *input,
     optr += 4;
     }
 
-  return (unsigned long)(optr - output);
+  return (size_t)(optr - output);
 }
 
 /*--------------------------------------------------------------------------*/
@@ -207,10 +207,10 @@ int kwsysBase64_Decode3(const unsigned char *src, unsigned char *dest)
    'length' parameter is ignored. This enables the caller to decode a stream
    without actually knowing how much decoded data to expect (of course, the
    buffer must be large enough). */
-unsigned long kwsysBase64_Decode(const unsigned char *input, 
-                                 unsigned long length,
-                                 unsigned char *output,
-                                 unsigned long max_input_length)
+size_t kwsysBase64_Decode(const unsigned char *input,
+                          size_t length,
+                          unsigned char *output,
+                          size_t max_input_length)
 {
   const unsigned char *ptr = input;
   unsigned char *optr = output;
@@ -226,7 +226,7 @@ unsigned long kwsysBase64_Decode(const unsigned char *input,
       optr += len;
       if(len < 3)
         {
-        return (unsigned long)(optr - output);
+        return (size_t)(optr - output);
         }
       ptr += 4;
       }
@@ -240,7 +240,7 @@ unsigned long kwsysBase64_Decode(const unsigned char *input,
       optr += len;
       if(len < 3)
         {
-        return (unsigned long)(optr - output);
+        return (size_t)(optr - output);
         }
       ptr += 4;
       }
@@ -275,5 +275,5 @@ unsigned long kwsysBase64_Decode(const unsigned char *input,
       }
     }
 
-  return (unsigned long)(optr - output);
+  return (size_t)(optr - output);
 }
diff --git a/Base64.h.in b/Base64.h.in
index 346800768..36ed3cce2 100644
--- a/Base64.h.in
+++ b/Base64.h.in
@@ -14,6 +14,8 @@
 
 #include <@KWSYS_NAMESPACE@/Configure.h>
 
+#include <stddef.h> /* size_t */
+
 /* Redefine all public interface symbol names to be in the proper
    namespace.  These macros are used internally to kwsys only, and are
    not visible to user code.  Use kwsysHeaderDump.pl to reproduce
@@ -68,10 +70,10 @@ kwsysEXPORT void kwsysBase64_Encode1(const unsigned char *src,
  * the extra padding needed to complete the encode 4 bytes will stop
  * the decoding anyway).
  */
-kwsysEXPORT unsigned long kwsysBase64_Encode(const unsigned char *input,
-                                             unsigned long length,
-                                             unsigned char *output,
-                                             int mark_end);
+kwsysEXPORT size_t kwsysBase64_Encode(const unsigned char *input,
+                                      size_t length,
+                                      unsigned char *output,
+                                      int mark_end);
 
 /**
  * Decode 4 bytes into a 3 byte string.  Returns the number of bytes
@@ -92,10 +94,10 @@ kwsysEXPORT int kwsysBase64_Decode3(const unsigned char *src,
  * much decoded data to expect (of course, the buffer must be large
  * enough).
  */
-kwsysEXPORT unsigned long kwsysBase64_Decode(const unsigned char *input, 
-                                             unsigned long length,
-                                             unsigned char *output,
-                                             unsigned long max_input_length);
+kwsysEXPORT size_t kwsysBase64_Decode(const unsigned char *input,
+                                      size_t length,
+                                      unsigned char *output,
+                                      size_t max_input_length);
 
 #if defined(__cplusplus)
 } /* extern "C" */