diff --git a/Makefile b/Makefile
index 94f7565d826..65b198f580a 100644
--- a/Makefile
+++ b/Makefile
@@ -68,7 +68,7 @@ debug:
 # for our stuff, kills optimization, and redirects to dev null so we
 # see any build failures.
 	make clean > /dev/null
-	CFLAGS='-g -O0' python3 -m pip -v install . > /dev/null
+	CFLAGS='-g -O2 -std=c99 -Wall -Wlogical-op' python3 -m pip -v install . > /dev/null
 
 .PHONY: release-test
 release-test:
diff --git a/setup.py b/setup.py
index 200fc92e9cc..d1817e07280 100644
--- a/setup.py
+++ b/setup.py
@@ -999,7 +999,7 @@ def debug_build():
 ext_modules = [
     Extension("PIL._imaging", files, libraries=["pil_imaging_mode"]),
     Extension("PIL._imagingft", ["src/_imagingft.c"], libraries=["pil_imaging_mode"]),
-    Extension("PIL._imagingcms", ["src/_imagingcms.c"]),
+    Extension("PIL._imagingcms", ["src/_imagingcms.c"], libraries=["pil_imaging_mode"]),
     Extension("PIL._webp", ["src/_webp.c"], libraries=["pil_imaging_mode"]),
     Extension("PIL._imagingtk", ["src/_imagingtk.c", "src/Tk/tkImaging.c"]),
     Extension("PIL._imagingmath", ["src/_imagingmath.c"]),
diff --git a/src/_imagingcms.c b/src/_imagingcms.c
index 2b9612db73d..4dff47aaf79 100644
--- a/src/_imagingcms.c
+++ b/src/_imagingcms.c
@@ -212,7 +212,8 @@ cms_transform_dealloc(CmsTransformObject *self) {
 /* internal functions */
 
 static cmsUInt32Number
-findLCMStype(char *PILmode) {
+findLCMStype(const char *PILmode) {
+    const ModeID mode = findModeID(PILmode);
     if (strcmp(PILmode, "RGB") == 0 || strcmp(PILmode, "RGBA") == 0 ||
         strcmp(PILmode, "RGBX") == 0) {
         return TYPE_RGBA_8;