1 files changed, 6 insertions, 1 deletions

diff --git a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
index ccfb630f..59ceae3f 100644
--- a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
+++ b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
@@ -521,7 +521,12 @@ public class ModelInitializer {
                 for (int i = 0; i < arity; i++) {
                         var argument = arguments.get(i);
                         if (argument instanceof NodeAssertionArgument nodeArgument) {
-                                nodes[i] = getNodeId(nodeArgument.getNode());
+                                var variableOrNode = nodeArgument.getNode();
+                                if (variableOrNode instanceof Node node) {
+                                        nodes[i] = getNodeId(node);
+                                } else {
+                                        throw new TracedException(argument, "Invalid assertion argument: " + variableOrNode);
+                                }
                         } else if (argument instanceof WildcardAssertionArgument) {
                                 nodes[i] = -1;
                         } else {