Index: src/OFMutableString.m
==================================================================
--- src/OFMutableString.m
+++ src/OFMutableString.m
@@ -213,10 +213,14 @@
 	string[length] = 0;
 }
 
 - (void)appendString: (OFString*)str
 {
+	if (str == nil)
+		@throw [OFInvalidArgumentException newWithClass: isa
+						       selector: _cmd];
+
 	[self appendCString: [str cString]];
 }
 
 - (void)appendFormat: (OFString*)fmt, ...
 {