diff --git a/core/src/main/java/org/projectnessie/cel/checker/Checker.java b/core/src/main/java/org/projectnessie/cel/checker/Checker.java
index 56bed6f8..70892363 100644
--- a/core/src/main/java/org/projectnessie/cel/checker/Checker.java
+++ b/core/src/main/java/org/projectnessie/cel/checker/Checker.java
@@ -408,6 +408,10 @@ OverloadResolution resolveOverload(
         // not a compatible call style.
         continue;
       }
+      if (overload.getParamsCount() != argTypes.size()) {
+        // not a compatible arity.
+        continue;
+      }
 
       Type overloadType = Decls.newFunctionType(overload.getResultType(), overload.getParamsList());
       if (overload.getTypeParamsCount() > 0) {
diff --git a/core/src/main/java/org/projectnessie/cel/checker/CheckerEnv.java b/core/src/main/java/org/projectnessie/cel/checker/CheckerEnv.java
index 55c3077a..6ed4258e 100644
--- a/core/src/main/java/org/projectnessie/cel/checker/CheckerEnv.java
+++ b/core/src/main/java/org/projectnessie/cel/checker/CheckerEnv.java
@@ -170,19 +170,28 @@ public Decl lookupFunction(String name) {
    */
   Decl addOverload(Decl f, Overload overload, List<String> errMsgs) {
     FunctionDecl function = f.getFunction();
-    Mapping emptyMappings = newMapping();
-    Type overloadFunction =
-        Decls.newFunctionType(overload.getResultType(), overload.getParamsList());
-    Type overloadErased = substitute(emptyMappings, overloadFunction, true);
+    Mapping emptyMappings = null;
+    Type overloadFunction = null;
+    Type overloadErased = null;
     boolean hasErr = false;
     for (Overload existing : function.getOverloadsList()) {
+      if (overload.getIsInstanceFunction() != existing.getIsInstanceFunction()
+          || overload.getParamsCount() != existing.getParamsCount()) {
+        continue;
+      }
+      if (emptyMappings == null) {
+        emptyMappings = newMapping();
+        overloadFunction =
+            Decls.newFunctionType(overload.getResultType(), overload.getParamsList());
+        overloadErased = substitute(emptyMappings, overloadFunction, true);
+      }
       Type existingFunction =
           Decls.newFunctionType(existing.getResultType(), existing.getParamsList());
       Type existingErased = substitute(emptyMappings, existingFunction, true);
       boolean overlap =
           isAssignable(emptyMappings, overloadErased, existingErased) != null
               || isAssignable(emptyMappings, existingErased, overloadErased) != null;
-      if (overlap && overload.getIsInstanceFunction() == existing.getIsInstanceFunction()) {
+      if (overlap) {
         errMsgs.add(
             overlappingOverloadError(
                 f.getName(),
diff --git a/core/src/test/java/org/projectnessie/cel/checker/CheckerEnvTest.java b/core/src/test/java/org/projectnessie/cel/checker/CheckerEnvTest.java
index 5ffe755b..f7bb0e47 100644
--- a/core/src/test/java/org/projectnessie/cel/checker/CheckerEnvTest.java
+++ b/core/src/test/java/org/projectnessie/cel/checker/CheckerEnvTest.java
@@ -20,6 +20,7 @@
 import static org.projectnessie.cel.common.types.pb.ProtoTypeRegistry.newRegistry;
 
 import com.google.api.expr.v1alpha1.Type;
+import java.util.Arrays;
 import java.util.List;
 import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.Test;
@@ -61,4 +62,20 @@ Overloads.ToDyn, singletonList(paramA), Decls.Dyn, typeParamAList))))
         .hasMessage(
             "overlapping overload for name 'dyn' (type '(type_param: \"A\") -> dyn' with overloadId: 'to_dyn' cannot be distinguished from '(type_param: \"A\") -> dyn' with overloadId: 'to_dyn')");
   }
+
+  @Test
+  void overloadsWithDifferentArityOrStyleDoNotOverlap() {
+    CheckerEnv env = newStandardCheckerEnv(Container.defaultContainer, newRegistry());
+
+    env.add(
+        Decls.newFunction(
+            "custom",
+            Decls.newOverload("custom_string", singletonList(Decls.String), Decls.String),
+            Decls.newOverload(
+                "custom_string_string", Arrays.asList(Decls.String, Decls.String), Decls.String),
+            Decls.newInstanceOverload(
+                "custom_receiver_string",
+                Arrays.asList(Decls.String, Decls.String),
+                Decls.String)));
+  }
 }