diff --git a/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/domain/ClientFamilyMembersRepository.java b/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/domain/ClientFamilyMembersRepository.java
index 4b43f73cc..e9614d5bd 100644
--- a/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/domain/ClientFamilyMembersRepository.java
+++ b/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/domain/ClientFamilyMembersRepository.java
@@ -28,16 +28,27 @@ public interface ClientFamilyMembersRepository
         extends JpaRepository<ClientFamilyMembers, Long>, JpaSpecificationExecutor<ClientFamilyMembers> {
 
     /**
-     * Counts non-deleted family members for a client that hold a given relationship code, optionally
-     * ignoring one specific family member (used at update time so a row doesn't conflict with itself).
-     * Backs the single-use relationship rule for Mother / Father / Spouse.
+     * Counts non-deleted family members for a client that hold a given relationship code.
+     * Backs the single-use relationship rule for Mother / Father / Spouse on create.
      */
     @Query("SELECT COUNT(fm) FROM ClientFamilyMembers fm "
             + "WHERE fm.client.id = :clientId "
             + "AND fm.relationship.id = :relationshipId "
-            + "AND fm.isDeleted = false "
-            + "AND (:excludeFamilyMemberId IS NULL OR fm.id <> :excludeFamilyMemberId)")
+            + "AND fm.isDeleted = false")
     long countActiveByClientAndRelationship(@Param("clientId") Long clientId,
+            @Param("relationshipId") Long relationshipId);
+
+    // Update variant: excludes one specific row so it doesn't conflict with itself.
+    // Why: PostgreSQL parses parameter types at parse-time; a nullable Long parameter compared with
+    // <> against a bigint column gets bound as varchar when null, producing
+    // "operator does not exist: bigint <> character varying". Splitting create vs. update keeps
+    // both queries non-nullable.
+    @Query("SELECT COUNT(fm) FROM ClientFamilyMembers fm "
+            + "WHERE fm.client.id = :clientId "
+            + "AND fm.relationship.id = :relationshipId "
+            + "AND fm.isDeleted = false "
+            + "AND fm.id <> :excludeFamilyMemberId")
+    long countActiveByClientAndRelationshipExcluding(@Param("clientId") Long clientId,
             @Param("relationshipId") Long relationshipId,
             @Param("excludeFamilyMemberId") Long excludeFamilyMemberId);
 
diff --git a/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/service/ClientFamilyMembersWritePlatformServiceImpl.java b/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/service/ClientFamilyMembersWritePlatformServiceImpl.java
index 62f941f83..eca15dc08 100644
--- a/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/service/ClientFamilyMembersWritePlatformServiceImpl.java
+++ b/fineract-provider/src/main/java/org/apache/fineract/portfolio/client/service/ClientFamilyMembersWritePlatformServiceImpl.java
@@ -99,8 +99,9 @@ public class ClientFamilyMembersWritePlatformServiceImpl implements ClientFamily
         if (!SINGLE_USE_RELATIONSHIPS.contains(label)) {
             return;
         }
-        final long existing = clientFamilyRepository.countActiveByClientAndRelationship(
-                clientId, relationship.getId(), excludeMemberId);
+        final long existing = (excludeMemberId == null)
+                ? clientFamilyRepository.countActiveByClientAndRelationship(clientId, relationship.getId())
+                : clientFamilyRepository.countActiveByClientAndRelationshipExcluding(clientId, relationship.getId(), excludeMemberId);
         if (existing > 0) {
             final String displayName = relationship.getLabel();
             throw new PlatformApiDataValidationException(java.util.List.of(ApiParameterError.parameterError(