diff --git a/lib/MC/MCParser/AsmLexer.cpp b/lib/MC/MCParser/AsmLexer.cpp
index e474a1501c8..5c617bc93d1 100644
--- a/lib/MC/MCParser/AsmLexer.cpp
+++ b/lib/MC/MCParser/AsmLexer.cpp
@@ -184,12 +184,12 @@ AsmToken AsmLexer::LexDigit() {
 
     long long Value;
     if (Result.getAsInteger(10, Value)) {
-      // We have to handle minint_as_a_positive_value specially, because
-      // - minint_as_a_positive_value = minint and it is valid.
-      if (Result == "9223372036854775808")
-        Value = -9223372036854775808ULL;
-      else
-        return ReturnError(TokStart, "Invalid decimal number");
+      // Allow positive values that are too large to fit into a signed 64-bit
+      // integer, but that do fit in an unsigned one, we just convert them over.
+      unsigned long long UValue;
+      if (Result.getAsInteger(10, UValue))
+        return ReturnError(TokStart, "invalid decimal number");
+      Value = (long long)UValue;
     }
     
     // The darwin/x86 (and x86-64) assembler accepts and ignores ULL and LL
diff --git a/test/MC/X86/x86-64.s b/test/MC/X86/x86-64.s
index a285c6f6456..5375919d55b 100644
--- a/test/MC/X86/x86-64.s
+++ b/test/MC/X86/x86-64.s
@@ -916,3 +916,7 @@ movq	%mm5, %rbx // CHECK: movd %mm5, %rbx # encoding: [0x48,0x0f,0x7e,0xeb]
 
 rex64 // CHECK: rex64 # encoding: [0x48]
 data16 // CHECK: data16 # encoding: [0x66]
+
+// PR8855
+movq 18446744073709551615,%rbx   // CHECK: movq	-1, %rbx
+