diff --git a/src/Value.c b/src/Value.c
index 267a4c99..200ccca8 100644
--- a/src/Value.c
+++ b/src/Value.c
@@ -139,7 +139,8 @@ static const char* isOnlyWhitespace(const char* value)
     }
     for(const char* c=value;*c!='\0'; c++)
     {
-        if(!isspace(*c))
+        // http://www.cplusplus.com/reference/cctype/isspace/
+        if(!isspace((unsigned char)*c))
         {
             return value;
         }