diff --git a/src/d_dehacked.cpp b/src/d_dehacked.cpp
index d05406910d..85a8e8c581 100644
--- a/src/d_dehacked.cpp
+++ b/src/d_dehacked.cpp
@@ -2030,7 +2030,7 @@ static int PatchPars (int dummy)
 		{
 			while ('\0' != *str)
 			{
-				if (isspace(*str))
+				if (isspace((unsigned char)*str))
 				{
 					return str;
 				}