diff --git a/src/evaluator/env.rs b/src/evaluator/env.rs
index 5c18da4..25b14d9 100644
--- a/src/evaluator/env.rs
+++ b/src/evaluator/env.rs
@@ -107,9 +107,7 @@ pub fn core_environment() -> Rc<Environment> {
         (
             "hashmap",
             Expression::NativeFunc(|args| {
-                if args.len() % 2 != 0 {
-                    Err(Error::MismatchedArgCount(2, args.len()))?
-                }
+                arg_count!(modulo: 2, args.len());
 
                 let mut index = -1;
                 let (keys, values): (Vec<_>, Vec<_>) = args.into_iter().partition(|_| {
diff --git a/src/evaluator/macros.rs b/src/evaluator/macros.rs
index 6a0c343..36d6532 100644
--- a/src/evaluator/macros.rs
+++ b/src/evaluator/macros.rs
@@ -5,6 +5,13 @@ macro_rules! arg_count {
             Err(Error::MismatchedArgCount($expected, $given))?
         }
     };
+
+    (modulo: $modulo:expr, $given:expr) => {
+        Err(Error::MismatchedArgCount(
+            ($given / $modulo) * $modulo + $modulo,
+            $given,
+        ))?
+    };
 }
 
 pub(crate) use arg_count;