diff --git a/Signal/src/util/StringUtil.m b/Signal/src/util/StringUtil.m
index 5012c2c7a..5159869fc 100644
--- a/Signal/src/util/StringUtil.m
+++ b/Signal/src/util/StringUtil.m
@@ -134,7 +134,14 @@
 -(NSNumber*) tryParseAsDecimalNumber {
     NSNumberFormatter* formatter = [NSNumberFormatter new];
     [formatter setNumberStyle:NSNumberFormatterDecimalStyle];
-    return [formatter numberFromString:self];
+
+    // NSNumberFormatter.numberFromString is good at noticing bad inputs, but loses precision for large values
+    // NSDecimalNumber.decimalNumberWithString has perfect precision, but lets bad inputs through sometimes (e.g. "88ffhih" -> 88)
+    // We use both to get both accuracy and detection of bad inputs
+    if ([formatter numberFromString:self] == nil) {
+        return nil;
+    }
+    return [NSDecimalNumber decimalNumberWithString:self];
 }
 -(NSNumber*) tryParseAsUnsignedInteger {
     NSNumber* value = [self tryParseAsDecimalNumber];
diff --git a/Signal/test/util/UtilTest.m b/Signal/test/util/UtilTest.m
index 230ba0aca..d2d21cef8 100644
--- a/Signal/test/util/UtilTest.m
+++ b/Signal/test/util/UtilTest.m
@@ -383,18 +383,25 @@
     test([@"88ffhih" tryParseAsUnsignedInteger] == nil);
     test([@"0xA" tryParseAsUnsignedInteger] == nil);
     test([@"A" tryParseAsUnsignedInteger] == nil);
-    test([@"4294967297" tryParseAsUnsignedInteger] == nil);
-    test([@"123456789123456789123456789123456789" tryParseAsUnsignedInteger] == nil);
     test([@"-1" tryParseAsUnsignedInteger] == nil);
     test([@"-" tryParseAsUnsignedInteger] == nil);
 
     test([[@"0" tryParseAsUnsignedInteger] isEqual:@0]);
+    test([[@"00" tryParseAsUnsignedInteger] isEqual:@0]);
     test([[@"1" tryParseAsUnsignedInteger] isEqual:@1]);
+    test([[@"01" tryParseAsUnsignedInteger] isEqual:@1]);
     test([[@"25" tryParseAsUnsignedInteger] isEqual:@25]);
     test([[(@NSUIntegerMax).description tryParseAsUnsignedInteger] isEqual:@NSUIntegerMax]);
-    if (NSUIntegerMax == 4294967295) {
+    if (NSUIntegerMax == 4294967295UL) {
         test([@"4294967296" tryParseAsUnsignedInteger] == nil);
     }
+    if (NSUIntegerMax == 18446744073709551615ULL) {
+        test([@"18446744073709551616" tryParseAsUnsignedInteger] == nil);
+    }
+
+    NSString* max = (@NSUIntegerMax).description;
+    NSString* farTooLarge = [max stringByAppendingString:max];
+    test([farTooLarge tryParseAsUnsignedInteger] == nil);
 }
 -(void) testRemoveAllCharactersIn {
     testThrows([@"" removeAllCharactersIn:nil]);