@@ -374,11 +374,11 @@
 
 	_s->cString[_s->cStringLength] = 0;
 
 	if ([string isKindOfClass: [OFUTF8String class]] ||
 	    [string isKindOfClass: [OFMutableUTF8String class]]) {
-		if (((OFUTF8String *)string)->_s->isUTF8)
+		if (((OFMutableUTF8String *)string)->_s->isUTF8)
 			_s->isUTF8 = true;
 	} else
 		_s->isUTF8 = true;
 }
 
@@ -549,11 +549,11 @@
 	_s->cStringLength = newCStringLength;
 	_s->length += string.length;
 
 	if ([string isKindOfClass: [OFUTF8String class]] ||
 	    [string isKindOfClass: [OFMutableUTF8String class]]) {
-		if (((OFUTF8String *)string)->_s->isUTF8)
+		if (((OFMutableUTF8String *)string)->_s->isUTF8)
 			_s->isUTF8 = true;
 	} else
 		_s->isUTF8 = true;
 }
 
@@ -642,11 +642,11 @@
 	_s->cStringLength = newCStringLength;
 	_s->length = newLength;
 
 	if ([replacement isKindOfClass: [OFUTF8String class]] ||
 	    [replacement isKindOfClass: [OFMutableUTF8String class]]) {
-		if (((OFUTF8String *)replacement)->_s->isUTF8)
+		if (((OFMutableUTF8String *)replacement)->_s->isUTF8)
 			_s->isUTF8 = true;
 	} else
 		_s->isUTF8 = true;
 }
 
@@ -729,11 +729,11 @@
 	_s->cStringLength = newCStringLength;
 	_s->length = newLength;
 
 	if ([replacement isKindOfClass: [OFUTF8String class]] ||
 	    [replacement isKindOfClass: [OFMutableUTF8String class]]) {
-		if (((OFUTF8String *)replacement)->_s->isUTF8)
+		if (((OFMutableUTF8String *)replacement)->_s->isUTF8)
 			_s->isUTF8 = true;
 	} else
 		_s->isUTF8 = true;
 }