diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 3ebe69c5bd5..8360fc9f414 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -102,6 +102,33 @@ uint64_t MCAsmLayout::getFragmentOffset(const MCFragment *F) const {
 }
 
 uint64_t MCAsmLayout::getSymbolOffset(const MCSymbolData *SD) const {
+  const MCSymbol &S = SD->getSymbol();
+
+  // If this is a variable, then recursively evaluate now.
+  if (S.isVariable()) {
+    MCValue Target;
+    if (!S.getVariableValue()->EvaluateAsRelocatable(Target, *this))
+      report_fatal_error("unable to evaluate offset for variable '" +
+                         S.getName() + "'");
+
+    // Verify that any used symbols are defined.
+    if (Target.getSymA() && Target.getSymA()->getSymbol().isUndefined())
+      report_fatal_error("unable to evaluate offset to undefined symbol '" +
+                         Target.getSymA()->getSymbol().getName() + "'");
+    if (Target.getSymB() && Target.getSymB()->getSymbol().isUndefined())
+      report_fatal_error("unable to evaluate offset to undefined symbol '" +
+                         Target.getSymB()->getSymbol().getName() + "'");
+      
+    uint64_t Offset = Target.getConstant();
+    if (Target.getSymA())
+      Offset += getSymbolOffset(&Assembler.getSymbolData(
+                                  Target.getSymA()->getSymbol()));
+    if (Target.getSymB())
+      Offset -= getSymbolOffset(&Assembler.getSymbolData(
+                                  Target.getSymB()->getSymbol()));
+    return Offset;
+  }
+
   assert(SD->getFragment() && "Invalid getOffset() on undefined symbol!");
   return getFragmentOffset(SD->getFragment()) + SD->getOffset();
 }
diff --git a/test/MC/MachO/variable-errors.s b/test/MC/MachO/variable-errors.s
new file mode 100644
index 00000000000..28308c691d9
--- /dev/null
+++ b/test/MC/MachO/variable-errors.s
@@ -0,0 +1,8 @@
+// RUN: not llvm-mc -triple x86_64-apple-darwin10 %s -filetype=obj -o %t.o 2> %t.err
+// RUN: FileCheck < %t.err %s
+
+        .data
+t0_a:
+t0_x = t0_a - t0_b
+// CHECK: unable to evaluate offset to undefined symbol 't0_b'
+	.long	t0_x