Run doxygen only if the doxygen directory exists
diff --git a/Makefile b/Makefile
index c1f6051..c18b99b 100644
--- a/Makefile
+++ b/Makefile
@@ -103,7 +103,7 @@
 
 apidoc:
 	mkdir -p apidoc
-	cd doxygen; doxygen mbedtls.doxyfile
+	cd doxygen && doxygen mbedtls.doxyfile
 
 apidoc_clean:
 	rm -rf apidoc