diff --git a/CHANGES/1217.contrib.rst b/CHANGES/1217.contrib.rst
new file mode 100644
index 000000000..082a09dd6
--- /dev/null
+++ b/CHANGES/1217.contrib.rst
@@ -0,0 +1 @@
+Fix compiler warnings and convert them to errors -- by :user:`asvetlov`.
diff --git a/multidict/_multidict.c b/multidict/_multidict.c
index f3ef01f5a..73c002296 100644
--- a/multidict/_multidict.c
+++ b/multidict/_multidict.c
@@ -139,7 +139,7 @@ _multidict_extend_parse_args(mod_state *state, PyObject *args, PyObject *kwds,
                              const char *name, PyObject **parg)
 {
     Py_ssize_t size = 0;
-    Py_ssize_t s;
+    Py_ssize_t s = 0;
     if (args) {
         s = PyTuple_GET_SIZE(args);
         if (s > 1) {
diff --git a/multidict/_multilib/hashtable.h b/multidict/_multilib/hashtable.h
index ce85ec8d7..8d1332acc 100644
--- a/multidict/_multilib/hashtable.h
+++ b/multidict/_multilib/hashtable.h
@@ -86,12 +86,12 @@ GROWTH_RATE(MultiDictObject *md)
     return md->used * 3;
 }
 
+#ifndef NDEBUG
 static inline int
 _md_check_consistency(MultiDictObject *md, bool update);
 static inline int
 _md_dump(MultiDictObject *md);
 
-#ifndef NDEBUG
 #define ASSERT_CONSISTENT(md, update) assert(_md_check_consistency(md, update))
 #else
 #define ASSERT_CONSISTENT(md, update) assert(1)
@@ -873,6 +873,8 @@ md_get_one(MultiDictObject *md, PyObject *key, PyObject **ret)
 static inline int
 md_get_all(MultiDictObject *md, PyObject *key, PyObject **ret)
 {
+    int tmp;
+    PyObject *value = NULL;
     *ret = NULL;
 
     md_finder_t finder = {0};
@@ -887,9 +889,6 @@ md_get_all(MultiDictObject *md, PyObject *key, PyObject **ret)
         goto fail;
     }
 
-    int tmp;
-    PyObject *value = NULL;
-
     while ((tmp = md_find_next(&finder, NULL, &value)) > 0) {
         if (*ret == NULL) {
             *ret = PyList_New(1);
@@ -1900,6 +1899,8 @@ md_clear(MultiDictObject *md)
     return 0;
 }
 
+#ifndef NDEBUG
+
 static inline int
 _md_check_consistency(MultiDictObject *md, bool update)
 {
@@ -1992,6 +1993,7 @@ _md_dump(MultiDictObject *md)
     printf("\n");
     return 1;
 }
+#endif  // NDEBUG
 
 #ifdef __cplusplus
 }
diff --git a/setup.py b/setup.py
index 33dd98afd..73b070d02 100644
--- a/setup.py
+++ b/setup.py
@@ -15,12 +15,13 @@
 if platform.system() != "Windows":
     CFLAGS.extend(
         [
-            "-std=c99",
+            "-std=c11",
             "-Wall",
             "-Wsign-compare",
             "-Wconversion",
             "-fno-strict-aliasing",
-            "-pedantic",
+            "-Wno-conversion",
+            "-Werror",
         ]
     )