diff --git a/llvm/lib/FileCheck/FileCheck.cpp b/llvm/lib/FileCheck/FileCheck.cpp
--- a/llvm/lib/FileCheck/FileCheck.cpp
+++ b/llvm/lib/FileCheck/FileCheck.cpp
@@ -146,14 +146,11 @@
   bool Hex = Value == Kind::HexUpper || Value == Kind::HexLower;
   uint64_t UnsignedValue;
   bool MissingFormPrefix = AlternateForm && !StrVal.consume_front("0x");
+  (void)MissingFormPrefix;
+  assert(!MissingFormPrefix && "missing alternate form prefix");
   if (StrVal.getAsInteger(Hex ? 16 : 10, UnsignedValue))
     return ErrorDiagnostic::get(SM, StrVal, IntegerParseErrorStr);
 
-  // Error out for a missing prefix only now that we know we have an otherwise
-  // valid integer.  For example, "-0x18" is reported above instead.
-  if (MissingFormPrefix)
-    return ErrorDiagnostic::get(SM, StrVal, "missing alternate form prefix");
-
   return ExpressionValue(UnsignedValue);
 }
 
diff --git a/llvm/unittests/FileCheck/FileCheckTest.cpp b/llvm/unittests/FileCheck/FileCheckTest.cpp
--- a/llvm/unittests/FileCheck/FileCheckTest.cpp
+++ b/llvm/unittests/FileCheck/FileCheckTest.cpp
@@ -311,7 +311,6 @@
     checkValueFromStringRepr("-5", -5);
     checkValueFromStringReprFailure(MaxUint64Str);
   } else {
-    checkValueFromStringReprFailure("-" + addBasePrefix("5"));
     checkValueFromStringRepr(addBasePrefix(MaxUint64Str), MaxUint64);
   }
 
@@ -321,9 +320,6 @@
   // Wrong casing is not tested because valueFromStringRepr() relies on
   // StringRef's getAsInteger() which does not allow to restrict casing.
   checkValueFromStringReprFailure(addBasePrefix("G"));
-
-  if (AlternateForm)
-    checkValueFromStringReprFailure("9", "missing alternate form prefix");
 }
 
 TEST_P(ExpressionFormatParameterisedFixture, FormatBoolOperator) {