diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 05ffe94..2699c12 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -915,7 +915,7 @@ impl<'a> Parser<'a> {
         }
         let loc = self.map.look_up_span(message.span);
         eprintln!(
-            "{}:{} Debug: {}",
+            "{}:{} DEBUG: {}",
             loc.file.name(),
             loc.begin.line + 1,
             message.node
diff --git a/src/value/number/mod.rs b/src/value/number/mod.rs
index e836e35..1662898 100644
--- a/src/value/number/mod.rs
+++ b/src/value/number/mod.rs
@@ -711,6 +711,10 @@ impl DivAssign for Number {
     }
 }
 
+fn modulo(n1: BigRational, n2: BigRational) -> BigRational {
+    (n1 % n2.clone() + n2.clone()) % n2
+}
+
 impl Rem for Number {
     type Output = Self;
 
@@ -721,25 +725,29 @@ impl Rem for Number {
                 Self::Small(val2) => {
                     let tuple1: (i64, i64) = val1.into();
                     let tuple2: (i64, i64) = val2.into();
-                    Self::Big(Box::new(
-                        BigRational::new_raw(BigInt::from(tuple1.0), BigInt::from(tuple1.1))
-                            % BigRational::new_raw(BigInt::from(tuple2.0), BigInt::from(tuple2.1)),
-                    ))
+
+                    Self::Big(Box::new(modulo(
+                        BigRational::new_raw(BigInt::from(tuple1.0), BigInt::from(tuple1.1)),
+                        BigRational::new_raw(BigInt::from(tuple2.0), BigInt::from(tuple2.1)),
+                    )))
                 }
                 Self::Big(val2) => {
                     let tuple: (i64, i64) = val1.into();
-                    Self::Big(Box::new(
-                        BigRational::new_raw(BigInt::from(tuple.0), BigInt::from(tuple.1)) % *val2,
-                    ))
+
+                    Self::Big(Box::new(modulo(
+                        BigRational::new_raw(BigInt::from(tuple.0), BigInt::from(tuple.1)),
+                        *val2,
+                    )))
                 }
             },
             Self::Big(val1) => match other {
-                Self::Big(val2) => Self::Big(Box::new(*val1 % *val2)),
+                Self::Big(val2) => Self::Big(Box::new(modulo(*val1, *val2))),
                 Self::Small(val2) => {
                     let tuple: (i64, i64) = val2.into();
-                    Self::Big(Box::new(
-                        *val1 % BigRational::new_raw(BigInt::from(tuple.0), BigInt::from(tuple.1)),
-                    ))
+                    Self::Big(Box::new(modulo(
+                        *val1,
+                        BigRational::new_raw(BigInt::from(tuple.0), BigInt::from(tuple.1)),
+                    )))
                 }
             },
         }
diff --git a/tests/modulo.rs b/tests/modulo.rs
index fb9abb8..87ee618 100644
--- a/tests/modulo.rs
+++ b/tests/modulo.rs
@@ -65,3 +65,23 @@ error!(
     positive_unit_mod_zero_unit_incompatible_units,
     "a {\n  color: 1rem % 0px;\n}\n", "Error: Incompatible units rem and px."
 );
+test!(
+    positive_mod_negative,
+    "a {\n  color: 1 % -4;\n}\n",
+    "a {\n  color: -3;\n}\n"
+);
+test!(
+    negative_mod_positive,
+    "a {\n  color: -4 % 3;\n}\n",
+    "a {\n  color: 2;\n}\n"
+);
+test!(
+    negative_mod_negative,
+    "a {\n  color: -4 % -3;\n}\n",
+    "a {\n  color: -1;\n}\n"
+);
+test!(
+    big_negative_mod_positive,
+    "a {\n  color: -99999990000099999999999999 % 2;\n}\n",
+    "a {\n  color: 1;\n}\n"
+);