diff --git a/src/js/ibcJS.js b/src/js/ibcJS.js
index 7ac1750f6a171a038573048d41b27ae5ff98f75a..2832cfb34ede5db8285cf395aad6d11f499cd160 100644
--- a/src/js/ibcJS.js
+++ b/src/js/ibcJS.js
@@ -196,6 +196,31 @@ globalThis.ibc = (() => {
 
 		slaves.forEach(s => create_node_rec(s));
 
+		let detectCycle = function(node, visited, recStack) { // doesn't have to be fast, will only be called after the stack explodes
+			if (!visited[node.id]) {
+				visited[node.id] = true;
+				recStack.push(node.id);
+				if (node.father) {
+					if (!visited[node.father.id] && detectCycle(node.father, visited, recStack)) {
+						return true;
+					} else if (recStack.includes(node.father.id)) {
+						recStack.push(node.father.id);
+						return true;
+					}
+				}
+				if (node.mother) {
+					if (!visited[node.mother.id] && detectCycle(node.mother, visited, recStack)) {
+						return true;
+					} else if (recStack.includes(node.mother.id)) {
+						recStack.push(node.mother.id);
+						return true;
+					}
+				}
+			}
+			recStack.pop();
+			return false;
+		}
+
 		let countSelfAndAncestors = function(node) {
 			if (node === null)
 				return 0 + 0;  // no self, no ancestors
@@ -205,8 +230,27 @@ globalThis.ibc = (() => {
 
 			return 1 + node._ancestorNodeCount;  // 1 self, maybe some ancestors
 		};
-		for (let slave of slaves)
-			countSelfAndAncestors(nodes[slave.ID]);
+		for (let slave of slaves) {
+			try {
+				countSelfAndAncestors(nodes[slave.ID]);
+			} catch (e) {
+				// probably a cycle, try to go find it and throw a better error
+				const visited = {};
+				const recStack = []; // track back edges in node graph
+				const IDToName = (id) => {
+					try {
+						return SlaveFullName(world.findSlaveState(id));
+					} catch {
+						return ``;
+					}
+				}
+				if (detectCycle(nodes[slave.ID], visited, recStack)) {
+					throw new Error(`Heritance cycle detected: ${toSentence(recStack.map(s => `${s} (${IDToName(s)})`), " was born to ", " was born to ")}, making them their own ancestor...`);
+				} else {
+					throw e; // not a cycle? no idea then
+				}
+			}
+		}
 
 		return nodes;
 	};