diff --git a/lib/preprocessor.cpp b/lib/preprocessor.cpp
index a9052ad15..1cfdd0b09 100644
--- a/lib/preprocessor.cpp
+++ b/lib/preprocessor.cpp
@@ -1432,7 +1432,7 @@ std::list<std::string> Preprocessor::getcfgs(const std::string &filedata, const
         }
 
         else if (line.compare(0, 5, "#else") == 0 && ! deflist.empty()) {
-            if (deflist.back() == "!") {
+            if (deflist.back() == "!" && !ndeflist.empty()) {
                 deflist.back() = ndeflist.back();
                 ndeflist.pop_back();
             } else {
@@ -1442,7 +1442,7 @@ std::list<std::string> Preprocessor::getcfgs(const std::string &filedata, const
         }
 
         else if (line.compare(0, 6, "#endif") == 0 && ! deflist.empty()) {
-            if (deflist.back() == "!")
+            if (deflist.back() == "!" && !ndeflist.empty())
                 ndeflist.pop_back();
             deflist.pop_back();
         }
diff --git a/test/testpreprocessor.cpp b/test/testpreprocessor.cpp
index e0f32b3d8..d98e7f90b 100644
--- a/test/testpreprocessor.cpp
+++ b/test/testpreprocessor.cpp
@@ -302,6 +302,7 @@ private:
         TEST_CASE(if_sizeof);
 
         TEST_CASE(double_include); // #5717
+        TEST_CASE(invalid_ifs)// #5909
     }
 
 
@@ -4042,6 +4043,27 @@ private:
         std::set<std::string> pragmaOnce;
         preprocessor.handleIncludes(code, "123.h", includePaths, defs, pragmaOnce, std::list<std::string>());
     }
+
+    void invalid_ifs()  {
+        const char filedata[] = "#ifdef\n"
+                                "#endif\n"
+                                "#ifdef !\n"
+                                "#endif\n"
+                                "#if defined\n"
+                                "#endif\n"
+                                "#define f(x) x\n"
+                                "#if f(2\n"
+                                "#endif\n"
+                                "int x;\n";
+
+        // Preprocess => don't crash..
+        std::istringstream istr(filedata);
+        std::map<std::string, std::string> actual;
+        Settings settings;
+        Preprocessor preprocessor(&settings, this);
+        preprocessor.preprocess(istr, actual, "file.c");
+
+    }
 };
 
 REGISTER_TEST(TestPreprocessor)