diff --git a/payloads/libpayload/drivers/video/graphics.c b/payloads/libpayload/drivers/video/graphics.c
index 632a283de9..b9a93a03e2 100644
--- a/payloads/libpayload/drivers/video/graphics.c
+++ b/payloads/libpayload/drivers/video/graphics.c
@@ -715,19 +715,20 @@ static fpmath_t lanczos_weight(fpmath_t in, int off)
 	 *
 	 * So (off - S0) - (in - floor(in)) is the distance from the sample
 	 * pixel to S0 minus the distance from S0 to the output pixel, aka
-	 * the distance from the sample pixel to the output pixel.
+	 * the distance from the sample pixel to the output pixel. (Note that
+	 * this calculation using fpfloor() is only valid if |in| is not
+	 * negative, which is always the case for our current code.)
 	 */
 	fpmath_t x = fpisub(off - S0, fpsubi(in, fpfloor(in)));
 
-	if (fpequals(x, fp(0)))
-		return fp(1);
-
 	/* x * 2 / a can save some instructions if a == 2 */
 	fpmath_t x2a = x;
 	if (LNCZ_A != 2)
 		x2a = fpmul(x, fpfrac(2, LNCZ_A));
 
 	fpmath_t x_times_pi = fpmul(x, fppi());
+	if (fpequals(x_times_pi, fp(0)))
+		return fp(1);
 
 	/*
 	 * Rather than using sinr(pi*x), we leverage the "one-based" sine
diff --git a/payloads/libpayload/tests/drivers/Makefile.mk b/payloads/libpayload/tests/drivers/Makefile.mk
index 7705473ef7..8d668c2e42 100644
--- a/payloads/libpayload/tests/drivers/Makefile.mk
+++ b/payloads/libpayload/tests/drivers/Makefile.mk
@@ -1,6 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
-tests-y += speaker-test
+tests-y += graphics-test speaker-test
+
+graphics-test-srcs += tests/drivers/graphics-test.c
+graphics-test-srcs += libc/fpmath.c
 
 speaker-test-srcs += tests/drivers/speaker-test.c
 speaker-test-mocks += inb
diff --git a/payloads/libpayload/tests/drivers/graphics-test.c b/payloads/libpayload/tests/drivers/graphics-test.c
new file mode 100644
index 0000000000..6e4d9b3528
--- /dev/null
+++ b/payloads/libpayload/tests/drivers/graphics-test.c
@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include "../drivers/video/graphics.c"
+
+#include <libpayload.h>
+#include <tests/test.h>
+
+static void test_lanczos(void **state)
+{
+	/* This makes sure the `x` in lanczos_weight() becomes very small. This
+	   test is mostly here to prove we won't end up dividing by zero. */
+	fpmath_t x = lanczos_weight(fpisub(1, fpfrac(1, 1 << 30)), S0 + 1);
+	assert_true(fpequals(x, fp(1)));
+}
+
+int main(void)
+{
+	const struct CMUnitTest tests[] = {
+		cmocka_unit_test(test_lanczos),
+	};
+
+	return lp_run_group_tests(tests, NULL, NULL);
+}