Index: src/OFMutableString.m
==================================================================
--- src/OFMutableString.m
+++ src/OFMutableString.m
@@ -523,10 +523,13 @@
 	const of_unichar_t *characters;
 	const of_unichar_t *searchCharacters = [string characters];
 	size_t searchLength = [string length];
 	size_t replacementLength = [replacement length];
 
+	if (string == nil || replacement == nil)
+		@throw [OFInvalidArgumentException exception];
+
 	if (range.length > SIZE_MAX - range.location ||
 	    range.location + range.length > [self length])
 		@throw [OFOutOfRangeException exception];
 
 	if (searchLength > range.length) {

Index: src/OFMutableString_UTF8.m
==================================================================
--- src/OFMutableString_UTF8.m
+++ src/OFMutableString_UTF8.m
@@ -593,10 +593,13 @@
 {
 	size_t start = range.location;
 	size_t end = range.location + range.length;
 	size_t newCStringLength, newLength;
 
+	if (replacement == nil)
+		@throw [OFInvalidArgumentException exception];
+
 	if (range.length > SIZE_MAX - range.location || end > _s->length)
 		@throw [OFOutOfRangeException exception];
 
 	newLength = _s->length - range.length + [replacement length];
 
@@ -651,10 +654,13 @@
 	size_t searchLength = [string UTF8StringLength];
 	size_t replacementLength = [replacement UTF8StringLength];
 	size_t last, newCStringLength, newLength;
 	char *newCString;
 
+	if (string == nil || replacement == nil)
+		@throw [OFInvalidArgumentException exception];
+
 	if (range.length > SIZE_MAX - range.location ||
 	    range.location + range.length > [self length])
 		@throw [OFOutOfRangeException exception];
 
 	if (_s->isUTF8) {