diff --git a/src/util/variant.c b/src/util/variant.c
index 74d7adb1c..2ffe4a4e3 100644
--- a/src/util/variant.c
+++ b/src/util/variant.c
@@ -49,6 +49,12 @@ variant frac_add(variant a, variant b)
     return frac_make(num, den);
 }
 
+variant frac_sub(variant a, variant b)
+{
+    b.sa[0] = -b.sa[0];
+    return frac_add(a, b);
+}
+
 variant frac_mul(variant a, variant b)
 {
     return frac_make(a.sa[0] * b.sa[0], a.sa[1] * b.sa[1]);
diff --git a/src/util/variant.h b/src/util/variant.h
index f98fa0b80..4bd5d952d 100644
--- a/src/util/variant.h
+++ b/src/util/variant.h
@@ -23,6 +23,7 @@ extern "C" {
 
   variant frac_make(int num, int den);
   variant frac_add(variant a, variant b);
+  variant frac_sub(variant a, variant b);
   variant frac_mul(variant a, variant b);
   variant frac_div(variant a, variant b);
 
diff --git a/src/util/variant.test.c b/src/util/variant.test.c
index 6344e5c33..793e4f683 100644
--- a/src/util/variant.test.c
+++ b/src/util/variant.test.c
@@ -18,6 +18,10 @@ static void test_fractions(CuTest *tc) {
     a = frac_div(b, b);
     CuAssertIntEquals(tc, 1, a.sa[0]);
     CuAssertIntEquals(tc, 1, a.sa[1]);
+    a = frac_sub(a, a);
+    CuAssertIntEquals(tc, 0, a.sa[0]);
+    a = frac_mul(a, b);
+    CuAssertIntEquals(tc, 0, a.sa[0]);
 }
 
 CuSuite *get_variant_suite(void)