diff --git a/lib/Makefile.am b/lib/Makefile.am
index d02bf02250..8a2bd4e68e 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -164,8 +164,8 @@ UNITV_ = $(UNITV_0)
 $(UNITPROTOS): $(CSOURCES)
 	$(UNIT_V)(cd $(srcdir) && @PERL@ ../scripts/extract-unit-protos $(CSOURCES)) > $(top_builddir)/lib/$(UNITPROTOS)
 
-_tidy_cflags :=
-TIDYFLAGS :=
+_tidy_cflags =
+TIDYFLAGS =
 if CURL_WERROR
 TIDYFLAGS += '--warnings-as-errors=*'
 endif
diff --git a/src/Makefile.am b/src/Makefile.am
index 77dbbe26e9..69554ee490 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -215,8 +215,8 @@ all-local: checksrc
 endif
 endif
 
-_tidy_cflags :=
-TIDYFLAGS :=
+_tidy_cflags =
+TIDYFLAGS =
 if CURL_WERROR
 TIDYFLAGS += '--warnings-as-errors=*'
 endif