diff --git a/plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/bugs/FormatDecode.java b/plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/bugs/FormatDecode.java
index 5d38ffa969cb..28cdf3b653c3 100644
--- a/plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/bugs/FormatDecode.java
+++ b/plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/bugs/FormatDecode.java
@@ -188,7 +188,7 @@ class FormatDecode {
           case 's': // formatted string (general)
           case 'S':
             checkFlags(flagBits, LEFT_JUSTIFY | ALTERNATE | PREVIOUS, specifier);
-            allowed = ALL_VALIDATOR;
+            allowed = (flagBits & ALTERNATE) != 0 ? new FormattableValidator(specifier) : ALL_VALIDATOR;
             break;
           case 'c': // unicode character
           case 'C':
@@ -390,6 +390,18 @@ class FormatDecode {
     }
   }
 
+  private static class FormattableValidator extends Validator {
+
+    public FormattableValidator(String specifier) {
+      super(specifier);
+    }
+
+    @Override
+    public boolean valid(PsiType type) {
+      return InheritanceUtil.isInheritor(type, "java.util.Formattable");
+    }
+  }
+
   private static class MultiValidator extends Validator {
     private final Set<Validator> validators = new HashSet<>(3);
 
diff --git a/plugins/InspectionGadgets/test/com/siyeh/igtest/bugs/malformed_format_string/MalformedFormatString.java b/plugins/InspectionGadgets/test/com/siyeh/igtest/bugs/malformed_format_string/MalformedFormatString.java
index 51343c6851ab..dd2591ad2013 100644
--- a/plugins/InspectionGadgets/test/com/siyeh/igtest/bugs/malformed_format_string/MalformedFormatString.java
+++ b/plugins/InspectionGadgets/test/com/siyeh/igtest/bugs/malformed_format_string/MalformedFormatString.java
@@ -3,6 +3,7 @@ package com.siyeh.igtest.bugs.malformed_format_string;
 import java.sql.Timestamp;
 import java.util.Date;
 import java.util.Locale;
+import java.util.Formattable;
 
 public class MalformedFormatString {
 
@@ -117,3 +118,15 @@ public class MalformedFormatString {
         System.out.printf("%o %<o", 15);
     }
 }
+class A {
+    void m(Formattable f) {
+        // each one fails, but inspection doesn't find the problems
+        String.format("%#s", <warning descr="Argument type 'int' does not match the type of the format specifier '%#s'">0</warning>);
+        String.format("%#s", <warning descr="Argument type 'float' does not match the type of the format specifier '%#s'">0.5f</warning>);
+        String.format("%#S", <warning descr="Argument type 'String' does not match the type of the format specifier '%#S'">"hello"</warning>);
+        String.format("%#S", <warning descr="Argument type 'int' does not match the type of the format specifier '%#S'">new Object().hashCode()</warning>);
+
+        // should be OK
+        String.format("%#s", f);
+    }
+}
diff --git a/plugins/InspectionGadgets/testsrc/com/siyeh/ig/bugs/MalformedFormatStringInspectionTest.java b/plugins/InspectionGadgets/testsrc/com/siyeh/ig/bugs/MalformedFormatStringInspectionTest.java
index 326b108c6b6c..ccbf25ebc808 100644
--- a/plugins/InspectionGadgets/testsrc/com/siyeh/ig/bugs/MalformedFormatStringInspectionTest.java
+++ b/plugins/InspectionGadgets/testsrc/com/siyeh/ig/bugs/MalformedFormatStringInspectionTest.java
@@ -1,3 +1,18 @@
+/*
+ * Copyright 2000-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 package com.siyeh.ig.bugs;
 
 import com.intellij.codeInspection.InspectionProfileEntry;
@@ -22,6 +37,11 @@ public class MalformedFormatStringInspectionTest extends LightInspectionTestCase
   @Override
   protected String[] getEnvironmentClasses() {
     return new String[] {
+      "package java.util;" +
+      "public interface Formattable {" +
+      "  void formatTo(Formatter formatter, int flags, int width, int precision);" +
+      "}",
+
       "package java.time.temporal;\n" +
       "public interface TemporalAccessor {\n" +
       "    boolean isSupported(TemporalField field);\n" +