diff --git a/ark/schema/structure/structure.ts b/ark/schema/structure/structure.ts
index fb36b151e..f14f4e83a 100644
--- a/ark/schema/structure/structure.ts
+++ b/ark/schema/structure/structure.ts
@@ -227,6 +227,10 @@ const implementation: nodeImplementationOf<Structure.Declaration> =
 				const lInner = { ...l.inner }
 				const rInner = { ...r.inner }
 				const disjointResult = new Disjoint()
+				// props an index signature narrows to once the other side's keys
+				// are known. they can't be added to the side they came from, where
+				// a prop with the same key would never be merged with them
+				const derived: nodeOfKind<Prop.Kind>[] = []
 				if (l.undeclared) {
 					const lKey = l.keyof()
 					for (const k of r.requiredKeys) {
@@ -250,18 +254,8 @@ const implementation: nodeImplementationOf<Structure.Declaration> =
 							const indexOverlap = intersectNodesRoot(lKey, n.signature, ctx.$)
 							if (indexOverlap instanceof Disjoint) return []
 							const normalized = normalizeIndex(indexOverlap, n.value, ctx.$)
-							if (normalized.required) {
-								rInner.required = conflatenate(
-									rInner.required,
-									normalized.required
-								)
-							}
-							if (normalized.optional) {
-								rInner.optional = conflatenate(
-									rInner.optional,
-									normalized.optional
-								)
-							}
+							derived.push(...(normalized.required ?? []))
+							derived.push(...(normalized.optional ?? []))
 							return normalized.index ?? []
 						})
 					}
@@ -289,18 +283,8 @@ const implementation: nodeImplementationOf<Structure.Declaration> =
 							const indexOverlap = intersectNodesRoot(rKey, n.signature, ctx.$)
 							if (indexOverlap instanceof Disjoint) return []
 							const normalized = normalizeIndex(indexOverlap, n.value, ctx.$)
-							if (normalized.required) {
-								lInner.required = conflatenate(
-									lInner.required,
-									normalized.required
-								)
-							}
-							if (normalized.optional) {
-								lInner.optional = conflatenate(
-									lInner.optional,
-									normalized.optional
-								)
-							}
+							derived.push(...(normalized.required ?? []))
+							derived.push(...(normalized.optional ?? []))
 
 							return normalized.index ?? []
 						})
@@ -320,7 +304,7 @@ const implementation: nodeImplementationOf<Structure.Declaration> =
 					kind: "structure",
 					baseInner,
 					l: flattenConstraints(lInner),
-					r: flattenConstraints(rInner),
+					r: [...flattenConstraints(rInner), ...derived],
 					roots: [],
 					ctx
 				})
diff --git a/ark/type/__tests__/objects/onUndeclaredKey.test.ts b/ark/type/__tests__/objects/onUndeclaredKey.test.ts
index c500bd6e6..775af685a 100644
--- a/ark/type/__tests__/objects/onUndeclaredKey.test.ts
+++ b/ark/type/__tests__/objects/onUndeclaredKey.test.ts
@@ -27,6 +27,15 @@ contextualize(() => {
 		})
 	})
 
+	it("merges a narrowed index signature with a declared key", () => {
+		const L = type({ a: "number", "[string]": "number" })
+		const R = type({ a: "number", b: "number", "+": "reject" })
+		attest(L.and(R).expression).snap(
+			"{ a: number, b: number, + (undeclared): reject }"
+		)
+		attest(R.and(L).expression).equals(L.and(R).expression)
+	})
+
 	describe("traversal", () => {
 		const getExtraneousB = () => ({ a: "ok", b: "why?" })