diff --git a/lib/checkio.cpp b/lib/checkio.cpp
index 7b6c0d5df..98cdb53d6 100644
--- a/lib/checkio.cpp
+++ b/lib/checkio.cpp
@@ -267,7 +267,8 @@ void CheckIO::checkFileUsage()
                             f.append_mode = Filepointer::APPEND_EX;
                         else
                             f.append_mode = Filepointer::APPEND;
-                    }
+                    } else
+                        f.append_mode = Filepointer::UNKNOWN_AM;
                     f.mode_indent = indent;
                     break;
                 case Filepointer::POSITIONING:
diff --git a/test/testio.cpp b/test/testio.cpp
index a4f8a0e05..1e2e00c43 100644
--- a/test/testio.cpp
+++ b/test/testio.cpp
@@ -637,6 +637,14 @@ private:
               "    fflush(f);\n"
               "}");
         ASSERT_EQUALS("", errout.str()); // #5578
+
+        check("void foo() {\n"
+              "    FILE* f = fopen(\"\", \"a\");\n"
+              "    fclose(f);\n"
+              "    f = fopen(\"\", \"r\");\n"
+              "    fseek(f, 0, SEEK_SET);\n"
+              "}");
+        ASSERT_EQUALS("", errout.str()); // #6566
     }
 
     void fflushOnInputStream() {