diff --git a/llvm/lib/Transforms/IPO/BlockExtractor.cpp b/llvm/lib/Transforms/IPO/BlockExtractor.cpp
--- a/llvm/lib/Transforms/IPO/BlockExtractor.cpp
+++ b/llvm/lib/Transforms/IPO/BlockExtractor.cpp
@@ -119,6 +119,8 @@
                /*KeepEmpty=*/false);
     if (LineSplit.empty())
       continue;
+    if (LineSplit.size()!=2)
+      report_fatal_error("Invalid line format, expecting lines like: 'funcname bb1[;bb2..]'");
     SmallVector<StringRef, 4> BBNames;
     LineSplit[1].split(BBNames, ';', /*MaxSplit=*/-1,
                        /*KeepEmpty=*/false);
diff --git a/llvm/test/Transforms/BlockExtractor/invalid-line.ll b/llvm/test/Transforms/BlockExtractor/invalid-line.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/BlockExtractor/invalid-line.ll
@@ -0,0 +1,9 @@
+; RUN: echo 'foo' > %t
+; RUN: not opt -S -extract-blocks -extract-blocks-file=%t %s 2>&1 | FileCheck %s
+
+; CHECK: Invalid line
+define void @bar() {
+bb:
+  ret void
+}
+