diff --git a/lib/MC/MCParser/AsmParser.cpp b/lib/MC/MCParser/AsmParser.cpp
index b7953c1a597..9c998ff796c 100644
--- a/lib/MC/MCParser/AsmParser.cpp
+++ b/lib/MC/MCParser/AsmParser.cpp
@@ -2456,6 +2456,10 @@ bool AsmParser::ParseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
     }
 
     Alignment = 1ULL << Alignment;
+  } else {
+    // Reject alignments that aren't a power of two, for gas compatibility.
+    if (!isPowerOf2_64(Alignment))
+      Error(AlignmentLoc, "alignment must be a power of 2");
   }
 
   // Diagnose non-sensical max bytes to align.
diff --git a/test/MC/AsmParser/align_invalid.s b/test/MC/AsmParser/align_invalid.s
new file mode 100644
index 00000000000..0d06d9423ce
--- /dev/null
+++ b/test/MC/AsmParser/align_invalid.s
@@ -0,0 +1,10 @@
+# RUN: llvm-mc -triple i386-linux-gnu < %s 2>&1 | FileCheck %s -check-prefix=ELF
+# RUN: llvm-mc -triple i386-apple-darwin < %s 2>&1 | FileCheck %s -check-prefix=DARWIN
+
+.align 3
+# ELF: error: alignment must be a power of 2
+# DARWIN-NOT: error
+
+.align 32
+# ELF-NOT: error
+# DARWIN: error: invalid alignment value