diff --git a/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/PbsFrontendCompiler.java b/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/PbsFrontendCompiler.java
index 375dbdc4..1f68eeef 100644
--- a/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/PbsFrontendCompiler.java
+++ b/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/PbsFrontendCompiler.java
@@ -158,6 +158,17 @@ public final class PbsFrontendCompiler {
                 intrinsicByMethodName.put(intrinsicSurface.sourceMethodName(), intrinsicSurface);
             }
         }
+        final var returnSlotsByCallableAndArity = new HashMap<String, Integer>();
+        for (final var declaredFn : ast.functions()) {
+            final var key = callableArityKey(declaredFn.name(), declaredFn.parameters().size());
+            final var retSlots = returnSlotsFor(declaredFn);
+            if (returnSlotsByCallableAndArity.containsKey(key)
+                    && !returnSlotsByCallableAndArity.get(key).equals(retSlots)) {
+                returnSlotsByCallableAndArity.put(key, null);
+            } else {
+                returnSlotsByCallableAndArity.put(key, retSlots);
+            }
+        }
 
         final var executableFunctions = new ArrayList<IRBackendExecutableFunction>(ast.functions().size());
         for (final var fn : ast.functions()) {
@@ -221,6 +232,8 @@ public final class PbsFrontendCompiler {
                         calleeName,
                         null,
                         null,
+                        callExpr.arguments().size(),
+                        returnSlotsByCallableAndArity.get(callableArityKey(calleeName, callExpr.arguments().size())),
                         callExpr.span()));
             }
             instructions.add(new IRBackendExecutableFunction.Instruction(
@@ -231,10 +244,7 @@ public final class PbsFrontendCompiler {
                     null,
                     fn.span()));
 
-            final var returnSlots = switch (fn.returnKind()) {
-                case INFERRED_UNIT, EXPLICIT_UNIT -> 0;
-                case PLAIN, RESULT -> 1;
-            };
+            final var returnSlots = returnSlotsFor(fn);
             final var start = safeToInt(fn.span().getStart());
             final var end = safeToInt(fn.span().getEnd());
             executableFunctions.add(new IRBackendExecutableFunction(
@@ -259,6 +269,19 @@ public final class PbsFrontendCompiler {
         return (canonicalName + "#" + canonicalVersion).hashCode();
     }
 
+    private int returnSlotsFor(final PbsAst.FunctionDecl functionDecl) {
+        return switch (functionDecl.returnKind()) {
+            case INFERRED_UNIT, EXPLICIT_UNIT -> 0;
+            case PLAIN, RESULT -> 1;
+        };
+    }
+
+    private String callableArityKey(
+            final String callableName,
+            final int arity) {
+        return callableName + "#" + arity;
+    }
+
     private int safeToInt(final long value) {
         if (value > Integer.MAX_VALUE) {
             return Integer.MAX_VALUE;
diff --git a/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/IRVMLoweringErrorCode.java b/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/IRVMLoweringErrorCode.java
index c27ac894..c5b977d3 100644
--- a/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/IRVMLoweringErrorCode.java
+++ b/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/IRVMLoweringErrorCode.java
@@ -3,5 +3,6 @@ package p.studio.compiler.backend.irvm;
 public enum IRVMLoweringErrorCode {
     LOWER_IRVM_EMPTY_EXECUTABLE_INPUT,
     LOWER_IRVM_MISSING_CALLEE,
+    LOWER_IRVM_CALL_ARG_SLOTS_MISMATCH,
+    LOWER_IRVM_CALL_RET_SLOTS_MISMATCH,
 }
-
diff --git a/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/LowerToIRVMService.java b/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/LowerToIRVMService.java
index 2f9e23d4..327ed287 100644
--- a/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/LowerToIRVMService.java
+++ b/prometeu-compiler/prometeu-build-pipeline/src/main/java/p/studio/compiler/backend/irvm/LowerToIRVMService.java
@@ -65,6 +65,25 @@ public class LowerToIRVMService {
                                     IRVMLoweringErrorCode.LOWER_IRVM_MISSING_CALLEE,
                                     "missing callee function: " + key);
                         }
+                        final var calleeFunction = ordered.get(calleeId);
+                        if (instr.expectedArgSlots() != null
+                                && instr.expectedArgSlots() != calleeFunction.paramSlots()) {
+                            throw new IRVMLoweringException(
+                                    IRVMLoweringErrorCode.LOWER_IRVM_CALL_ARG_SLOTS_MISMATCH,
+                                    "call arg_slots mismatch for %s. expected=%d actual=%d".formatted(
+                                            key,
+                                            instr.expectedArgSlots(),
+                                            calleeFunction.paramSlots()));
+                        }
+                        if (instr.expectedRetSlots() != null
+                                && instr.expectedRetSlots() != calleeFunction.returnSlots()) {
+                            throw new IRVMLoweringException(
+                                    IRVMLoweringErrorCode.LOWER_IRVM_CALL_RET_SLOTS_MISMATCH,
+                                    "call ret_slots mismatch for %s. expected=%d actual=%d".formatted(
+                                            key,
+                                            instr.expectedRetSlots(),
+                                            calleeFunction.returnSlots()));
+                        }
                         instructions.add(new IRVMInstruction(IRVMOp.CALL, calleeId));
                         operations.add(BytecodeEmitter.Operation.callFunc(calleeId));
                     }
@@ -142,4 +161,3 @@ public class LowerToIRVMService {
         return (moduleKey == null ? "" : moduleKey) + "::" + callableName;
     }
 }
-
diff --git a/prometeu-compiler/prometeu-build-pipeline/src/test/java/p/studio/compiler/backend/irvm/LowerToIRVMServiceTest.java b/prometeu-compiler/prometeu-build-pipeline/src/test/java/p/studio/compiler/backend/irvm/LowerToIRVMServiceTest.java
index 20e887ac..dae3f3f3 100644
--- a/prometeu-compiler/prometeu-build-pipeline/src/test/java/p/studio/compiler/backend/irvm/LowerToIRVMServiceTest.java
+++ b/prometeu-compiler/prometeu-build-pipeline/src/test/java/p/studio/compiler/backend/irvm/LowerToIRVMServiceTest.java
@@ -79,6 +79,20 @@ class LowerToIRVMServiceTest {
         assertEquals(IRVMLoweringErrorCode.LOWER_IRVM_MISSING_CALLEE, thrown.code());
     }
 
+    @Test
+    void lowerMustRejectCallArgSlotMismatch() {
+        final var backend = IRBackend.builder()
+                .executableFunctions(ReadOnlyList.from(
+                        fn("callee", "app", ReadOnlyList.from(ret())),
+                        fn("main", "app", ReadOnlyList.from(
+                                callFuncWithExpected("app", "callee", 2, 0),
+                                ret()))))
+                .build();
+
+        final var thrown = assertThrows(IRVMLoweringException.class, () -> new LowerToIRVMService().lower(backend));
+        assertEquals(IRVMLoweringErrorCode.LOWER_IRVM_CALL_ARG_SLOTS_MISMATCH, thrown.code());
+    }
+
     private static IRBackendExecutableFunction fn(
             final String name,
             final String moduleKey,
@@ -117,6 +131,22 @@ class LowerToIRVMServiceTest {
                 Span.none());
     }
 
+    private static IRBackendExecutableFunction.Instruction callFuncWithExpected(
+            final String moduleKey,
+            final String name,
+            final int expectedArgSlots,
+            final int expectedRetSlots) {
+        return new IRBackendExecutableFunction.Instruction(
+                IRBackendExecutableFunction.InstructionKind.CALL_FUNC,
+                moduleKey,
+                name,
+                null,
+                null,
+                expectedArgSlots,
+                expectedRetSlots,
+                Span.none());
+    }
+
     private static IRBackendExecutableFunction.Instruction callHost(
             final String module,
             final String name,
@@ -145,4 +175,3 @@ class LowerToIRVMServiceTest {
                 Span.none());
     }
 }
-