diff --git a/Makefile b/Makefile
index f35b5318..cff224b7 100644
--- a/Makefile
+++ b/Makefile
@@ -114,6 +114,7 @@ $(APP_OBJS): CFLAGS += \
 	-Ideps/sqlite \
 	-Ideps/valgrind \
 	-Ideps/xopt \
+	-Wdouble-promotion \
 	-Werror
 
 BLOWFISH_SOURCES := \
diff --git a/src/ssb.tests.c b/src/ssb.tests.c
index 9a32fc56..e355f1cb 100644
--- a/src/ssb.tests.c
+++ b/src/ssb.tests.c
@@ -542,7 +542,7 @@ void tf_ssb_test_bench(const tf_test_options_t* options)
 		tf_ssb_append_post(ssb0, "Hello, world!");
 	}
 	clock_gettime(CLOCK_REALTIME, &end_time);
-	tf_printf("insert = %f seconds\n", (float)(end_time.tv_sec - start_time.tv_sec) + (end_time.tv_nsec - start_time.tv_nsec) / 1e9f);
+	tf_printf("insert = %f seconds\n", (end_time.tv_sec - start_time.tv_sec) + (end_time.tv_nsec - start_time.tv_nsec) / 1e9);
 
 	tf_ssb_t* ssb1 = tf_ssb_create(&loop, NULL, ":memory:");
 	tf_ssb_generate_keys(ssb1);
@@ -571,7 +571,7 @@ void tf_ssb_test_bench(const tf_test_options_t* options)
 	}
 	clock_gettime(CLOCK_REALTIME, &end_time);
 	tf_printf("Done.\n");
-	tf_printf("replicate = %f seconds\n", (float)(end_time.tv_sec - start_time.tv_sec) + (end_time.tv_nsec - start_time.tv_nsec) / 1e9f);
+	tf_printf("replicate = %f seconds\n", (end_time.tv_sec - start_time.tv_sec) + (end_time.tv_nsec - start_time.tv_nsec) / 1e9);
 
 	tf_ssb_send_close(ssb1);
 	tf_ssb_server_close(ssb0);