diff --git a/AFNetworking/AFHTTPClient.m b/AFNetworking/AFHTTPClient.m
index 5ff7343..bbf75d6 100644
--- a/AFNetworking/AFHTTPClient.m
+++ b/AFNetworking/AFHTTPClient.m
@@ -98,8 +98,15 @@ static NSString * AFBase64EncodedStringFromString(NSString *string) {
 NSString * AFURLEncodedStringFromStringWithEncoding(NSString *string, NSStringEncoding encoding) {
     static NSString * const kAFLegalCharactersToBeEscaped = @"?!@#$^&%*+,:;='\"`<>()[]{}/\\|~ ";
     
-    // Following the suggestion in documentation for `CFURLCreateStringByAddingPercentEscapes` to "pre-process" URL strings (using stringByReplacingPercentEscapesUsingEncoding) with unpredictable sequences that may already contain percent escapes.
-	return [(NSString *)CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (CFStringRef)[string stringByReplacingPercentEscapesUsingEncoding:encoding], NULL, (CFStringRef)kAFLegalCharactersToBeEscaped, CFStringConvertNSStringEncodingToEncoding(encoding)) autorelease];
+    /* 
+     The documentation for `CFURLCreateStringByAddingPercentEscapes` suggests that one should "pre-process" URL strings with unpredictable sequences that may already contain percent escapes.       However, if the string contains an unescaped sequence with '%' appearing without an escape code (such as when representing percentages like "42%"), `stringByReplacingPercentEscapesUsingEncoding` will return `nil`. Thus, the string is only unescaped if there are no invalid percent-escaped sequences. 
+    */
+    NSString *unescapedString = [string stringByReplacingPercentEscapesUsingEncoding:encoding];
+    if (unescapedString) {
+        string = unescapedString;
+    }
+    
+	return [(NSString *)CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (CFStringRef)string, NULL, (CFStringRef)kAFLegalCharactersToBeEscaped, CFStringConvertNSStringEncodingToEncoding(encoding)) autorelease];
 }
 
 NSString * AFQueryStringFromParametersWithEncoding(NSDictionary *parameters, NSStringEncoding encoding) {