diff --git a/lib/CodeGen/MIRParser/MIRParser.cpp b/lib/CodeGen/MIRParser/MIRParser.cpp
index 20d7f815655..1fef3f6dcb3 100644
--- a/lib/CodeGen/MIRParser/MIRParser.cpp
+++ b/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -212,7 +212,9 @@ bool MIRParserImpl::initializeMachineFunction(MachineFunction &MF) {
     if (!YamlMBB.Name.empty()) {
       BB = dyn_cast_or_null<BasicBlock>(
           F.getValueSymbolTable().lookup(YamlMBB.Name));
-      // TODO: Report an error if a basic block isn't found.
+      if (!BB)
+        return error(Twine("basic block '") + YamlMBB.Name +
+                     "' is not defined in the function '" + MF.getName() + "'");
     }
     auto *MBB = MF.CreateMachineBasicBlock(BB);
     MF.insert(MF.end(), MBB);
diff --git a/test/CodeGen/MIR/machine-basic-block-unknown-name.mir b/test/CodeGen/MIR/machine-basic-block-unknown-name.mir
new file mode 100644
index 00000000000..4c363c69edb
--- /dev/null
+++ b/test/CodeGen/MIR/machine-basic-block-unknown-name.mir
@@ -0,0 +1,18 @@
+# RUN: not llc -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that an error is reported whenever the MIR parser can't find
+# a basic block with the machine basis block's name.
+
+--- |
+
+  define i32 @foo() {
+  entry:
+    ret i32 0
+  }
+
+...
+---
+name:            foo
+body:
+  # CHECK: basic block 'entrie' is not defined in the function 'foo'
+  - name:         entrie
+...