diff --git a/lib/src/main/java/graphql/nadel/validation/NadelHydrationValidation.kt b/lib/src/main/java/graphql/nadel/validation/NadelHydrationValidation.kt
index 79aa90969..ec660b7da 100644
--- a/lib/src/main/java/graphql/nadel/validation/NadelHydrationValidation.kt
+++ b/lib/src/main/java/graphql/nadel/validation/NadelHydrationValidation.kt
@@ -98,7 +98,7 @@ internal class NadelHydrationValidation(
         hydrations: List<UnderlyingServiceHydration>,
     ): List<NadelSchemaValidationError> {
         if (hydrations.size > 1) {
-            val hasListSourceInputField = hydrations
+            val anyListSourceInputField = hydrations
                 .any { hydration ->
                     val parentType = parent.underlying as GraphQLFieldsContainer
                     hydration
@@ -112,7 +112,7 @@ internal class NadelHydrationValidation(
                         }
                 }
 
-            if (hasListSourceInputField) {
+            if (anyListSourceInputField) {
                 val sourceFields = hydrations
                     .flatMapTo(LinkedHashSet()) { hydration ->
                         hydration.arguments
diff --git a/lib/src/test/kotlin/graphql/nadel/validation/NadelHydrationValidationTest2.kt b/lib/src/test/kotlin/graphql/nadel/validation/NadelHydrationValidationTest2.kt
index fa32fb850..d8c2265d2 100644
--- a/lib/src/test/kotlin/graphql/nadel/validation/NadelHydrationValidationTest2.kt
+++ b/lib/src/test/kotlin/graphql/nadel/validation/NadelHydrationValidationTest2.kt
@@ -170,6 +170,94 @@ class NadelHydrationValidationTest2 {
         assertTrue(errors.single().subject.name == "data")
     }
 
+    @Test
+    fun `prohibit mixing list and non-list source input fields`() {
+        val fixture = NadelValidationTestFixture(
+            overallSchema = mapOf(
+                "activity" to /* language=GraphQL*/ """
+                    type Query {
+                        myActivity: [Activity]
+                    }
+                    union ActivityContent = User | Issue
+                    type Activity {
+                        id: ID!
+                        data: [ActivityContent]
+                        @hydrated(
+                            service: "users"
+                            field: "usersByIds"
+                            arguments: [
+                                {name: "ids", value: "$source.userIds"}
+                            ]
+                        )
+                        @hydrated(
+                            service: "issues"
+                            field: "issuesByIds"
+                            arguments: [
+                                {name: "ids", value: "$source.issueId"}
+                            ]
+                        )
+                    }
+                """.trimIndent(),
+                "users" to /* language=GraphQL*/ """
+                    type Query {
+                        usersByIds(ids: [ID]!): [User]
+                    }
+                    type User {
+                        id: ID!
+                        name: String!
+                    }
+                """.trimIndent(),
+                "issues" to /* language=GraphQL*/ """
+                    type Query {
+                        issuesByIds(ids: [ID]!): [Issue]
+                    }
+                    type Issue {
+                        id: ID!
+                        key: String
+                    }
+                """.trimIndent(),
+            ),
+            underlyingSchema = mapOf(
+                "activity" to /* language=GraphQL*/ """
+                    type Query {
+                        myActivity: [Activity]
+                    }
+                    type Activity {
+                        id: ID!
+                        userIds: [ID]
+                        issueId: ID
+                    }
+                """.trimIndent(),
+                "users" to /* language=GraphQL*/ """
+                    type Query {
+                        usersByIds(ids: [ID]!): [User]
+                    }
+                    type User {
+                        id: ID!
+                        name: String!
+                    }
+                    type Account {
+                        id: ID!
+                    }
+                """.trimIndent(),
+                "issues" to /* language=GraphQL*/ """
+                    type Query {
+                        issuesByIds(ids: [ID]!): [Issue]
+                    }
+                    type Issue {
+                        id: ID!
+                        key: String
+                    }
+                """.trimIndent(),
+            ),
+        )
+
+        val errors = validate(fixture)
+        assertTrue(errors.map { it.message }.isNotEmpty())
+        assertTrue(errors.single() is NadelSchemaValidationError.MultipleHydrationSourceInputFields)
+        assertTrue(errors.single().subject.name == "data")
+    }
+
     @Test
     fun `permit multiple source fields if source input field is not list type`() {
         val fixture = NadelValidationTestFixture(