diff --git a/Source/Additions/NSString+GNUstepBase.m b/Source/Additions/NSString+GNUstepBase.m
index 8a447ad9b..c970fc3fb 100644
--- a/Source/Additions/NSString+GNUstepBase.m
+++ b/Source/Additions/NSString+GNUstepBase.m
@@ -24,6 +24,7 @@
 */
 #import "common.h"
 #include <string.h>
+#include <ctype.h>
 #import "Foundation/NSException.h"
 #import "GNUstepBase/NSString+GNUstepBase.h"
 #import "GNUstepBase/NSMutableString+GNUstepBase.h"