diff --git a/AFNetworking/AFHTTPRequestOperation.m b/AFNetworking/AFHTTPRequestOperation.m
index cb57cdd..be49c21 100644
--- a/AFNetworking/AFHTTPRequestOperation.m
+++ b/AFNetworking/AFHTTPRequestOperation.m
@@ -58,6 +58,10 @@ NSSet * AFContentTypesFromHTTPHeader(NSString *string) {
 }
 
 static void AFGetMediaTypeAndSubtypeWithString(NSString *string, NSString **type, NSString **subtype) {
+    if (!string) {
+        return;
+    }
+
     NSScanner *scanner = [NSScanner scannerWithString:string];
     [scanner setCharactersToBeSkipped:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
     [scanner scanUpToString:@"/" intoString:type];