Index: src/OFMethodSignature.m
==================================================================
--- src/OFMethodSignature.m
+++ src/OFMethodSignature.m
@@ -40,11 +40,11 @@
 	assert(*length > 0);
 
 	(*type)++;
 	(*length)--;
 
-	while (*length > 0 && isdigit(**type)) {
+	while (*length > 0 && of_ascii_isdigit(**type)) {
 		(*type)++;
 		(*length)--;
 	}
 
 	align = alignofEncoding(type, length, true);
@@ -292,11 +292,11 @@
 	assert(*length > 0);
 
 	(*type)++;
 	(*length)--;
 
-	while (*length > 0 && isdigit(**type)) {
+	while (*length > 0 && of_ascii_isdigit(**type)) {
 		count = count * 10 + **type - '0';
 
 		(*type)++;
 		(*length)--;
 	}
@@ -605,11 +605,11 @@
 		_offsets = [[OFMutableData alloc]
 		    initWithItemSize: sizeof(size_t)];
 
 		last = _types;
 		for (size_t i = 0; i < length; i++) {
-			if (isdigit(_types[i])) {
+			if (of_ascii_isdigit(_types[i])) {
 				size_t offset = _types[i] - '0';
 
 				if (last == _types + i)
 					@throw [OFInvalidFormatException
 					    exception];
@@ -616,11 +616,12 @@
 
 				_types[i] = '\0';
 				[_typesPointers addItem: &last];
 
 				i++;
-				for (; i < length && isdigit(_types[i]); i++)
+				for (; i < length &&
+				    of_ascii_isdigit(_types[i]); i++)
 					offset = offset * 10 + _types[i] - '0';
 
 				[_offsets addItem: &offset];
 
 				last = _types + i;

Index: src/macros.h
==================================================================
--- src/macros.h
+++ src/macros.h
@@ -828,15 +828,21 @@
 static OF_INLINE bool
 of_ascii_isalpha(char c)
 {
 	return ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'));
 }
+
+static OF_INLINE bool
+of_ascii_isdigit(char c)
+{
+	return (c >= '0' && c <= '9');
+}
 
 static OF_INLINE bool
 of_ascii_isalnum(char c)
 {
-	return (of_ascii_isalpha(c) || (c >= '0' && c <= '9'));
+	return (of_ascii_isalpha(c) || of_ascii_isdigit(c));
 }
 
 static OF_INLINE bool
 of_ascii_isspace(char c)
 {