Index: lib/Frontend/SerializedDiagnosticReader.cpp
===================================================================
--- lib/Frontend/SerializedDiagnosticReader.cpp
+++ lib/Frontend/SerializedDiagnosticReader.cpp
@@ -27,6 +27,9 @@
   llvm::BitstreamCursor Stream(**Buffer);
   Optional<llvm::BitstreamBlockInfo> BlockInfo;
 
+  if (Stream.AtEndOfStream())
+    return SDError::InvalidSignature;
+
   // Sniff for the signature.
   if (Stream.Read(8) != 'D' ||
       Stream.Read(8) != 'I' ||
Index: test/Index/read-empty-diags.test
===================================================================
--- /dev/null
+++ test/Index/read-empty-diags.test
@@ -0,0 +1,2 @@
+// RUN: not c-index-test -read-diagnostics %S/Inputs/empty.dia 2>&1 | FileCheck %s
+// CHECK: Trouble deserializing file (Invalid File): Invalid diagnostics signature