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
@@ -7780,13 +7780,16 @@
       // }
       if (HasVAListArg) {
         if (const ParmVarDecl *PV = dyn_cast<ParmVarDecl>(VD)) {
-          if (const NamedDecl *ND = dyn_cast<NamedDecl>(PV->getDeclContext())) {
-            int PVIndex = PV->getFunctionScopeIndex() + 1;
-            for (const auto *PVFormat : ND->specific_attrs<FormatAttr>()) {
+          if (const Decl *D = dyn_cast<Decl>(PV->getDeclContext())) {
+            int PVIndex = 1;
+            if (const NamedDecl *ND = dyn_cast<NamedDecl>(D)) {
+              PVIndex += PV->getFunctionScopeIndex();
               // adjust for implicit parameter
               if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(ND))
                 if (MD->isInstance())
                   ++PVIndex;
+            }
+            for (const auto *PVFormat : D->specific_attrs<FormatAttr>()) {
               // We also check if the formats are compatible.
               // We can't pass a 'scanf' string to a 'printf' function.
               if (PVIndex == PVFormat->getFormatIdx() &&
diff --git a/clang/test/Sema/format-strings.c b/clang/test/Sema/format-strings.c
--- a/clang/test/Sema/format-strings.c
+++ b/clang/test/Sema/format-strings.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs %s
-// RUN: %clang_cc1 -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs -fno-signed-char %s
+// RUN: %clang_cc1 -fblocks -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs %s
+// RUN: %clang_cc1 -fblocks -fsyntax-only -verify -Wformat-nonliteral -isystem %S/Inputs -fno-signed-char %s
 
 #include <stdarg.h>
 #include <stddef.h>
@@ -714,3 +714,16 @@
 void test_printf_opaque_ptr(void *op) {
   printf("%s", op); // expected-warning{{format specifies type 'char *' but the argument has type 'void *'}}
 }
+
+void test_block() {
+  void __attribute__((__format__(__printf__, 1, 2))) (^printf_block)(const char *, ...) =
+      ^(const char *fmt, ...) __attribute__((__format__(__printf__, 1, 2))) {
+    va_list ap;
+    va_start(ap, fmt);
+    vprintf(fmt, ap);
+    va_end(ap);
+  };
+
+  printf_block("%s string %i\n", "aaa", 123);
+  printf_block("%s string\n", 123); // expected-warning{{format specifies type 'char *' but the argument has type 'int'}}
+}