diff --git a/Source/cmStringCommand.cxx b/Source/cmStringCommand.cxx
index eacbfedbee..aa24a02586 100644
--- a/Source/cmStringCommand.cxx
+++ b/Source/cmStringCommand.cxx
@@ -555,7 +555,7 @@ bool cmStringCommand::HandleSubstringCommand(std::vector<std::string> const& arg
     return false;
     }
   int leftOverLength = intStringLength - begin;
-  if ( end < 0 || end > intStringLength )
+  if ( end < 0 || end > leftOverLength )
     {
     cmOStringStream ostr;
     ostr << "end index: " << end << " is out of range " << 0 << " - " << leftOverLength;
diff --git a/Source/cmStringCommand.h b/Source/cmStringCommand.h
index 8c6a11ca4f..fc846d37cc 100644
--- a/Source/cmStringCommand.h
+++ b/Source/cmStringCommand.h
@@ -84,7 +84,7 @@ public:
       "  STRING(TOUPPER <string1> <output variable>)\n"
       "  STRING(TOLOWER <string1> <output variable>)\n"
       "  STRING(LENGTH <string> <output variable>)\n"
-      "  STRING(SUBSTRING <string> <begin> <end> <output variable>)\n"
+      "  STRING(SUBSTRING <string> <begin> <length> <output variable>)\n"
       "REGEX MATCH will match the regular expression once and store the "
       "match in the output variable.\n"
       "REGEX MATCHALL will match the regular expression as many times as "