diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -1066,6 +1066,9 @@
       return llvm::None;
     unsigned NewIndex = *IndexOptional;
 
+    if (NewIndex >= TheCall->getNumArgs())
+      return llvm::None;
+
     const Expr *ObjArg = TheCall->getArg(NewIndex);
     uint64_t Result;
     if (!ObjArg->tryEvaluateObjectSize(Result, getASTContext(), BOSType))
diff --git a/clang/test/Sema/format-strings-scanf.c b/clang/test/Sema/format-strings-scanf.c
--- a/clang/test/Sema/format-strings-scanf.c
+++ b/clang/test/Sema/format-strings-scanf.c
@@ -69,6 +69,11 @@
   scanf("%#.2Lf", ld); // expected-warning{{invalid conversion specifier '#'}}
 }
 
+void missing_argument_with_length_modifier() {
+  char buf[30];
+  scanf("%s:%900s", buf); // expected-warning{{more '%' conversions than data arguments}}
+}
+
 // Test that the scanf call site is where the warning is attached.  If the
 // format string is somewhere else, point to it in a note.
 void pr9751(void) {