diff --git a/source/java/org/alfresco/rest/api/impl/SitesImpl.java b/source/java/org/alfresco/rest/api/impl/SitesImpl.java
index 0c114e64d8..1ccdb8af23 100644
--- a/source/java/org/alfresco/rest/api/impl/SitesImpl.java
+++ b/source/java/org/alfresco/rest/api/impl/SitesImpl.java
@@ -406,6 +406,12 @@ public class SitesImpl implements Sites
     		throw new InvalidArgumentException("Must provide a role");
     	}
 
+    	/* MNT-10551 : fix */
+    	if (!siteService.isMember(siteId, siteMember.getPersonId()))
+    	{
+    		throw new InvalidArgumentException("User is not a member of the site");
+    	}
+
     	siteService.setMembership(siteId, siteMember.getPersonId(), siteRole.toString());
     	return siteMember;
 	}
diff --git a/source/test-java/org/alfresco/rest/api/tests/TestSiteMembers.java b/source/test-java/org/alfresco/rest/api/tests/TestSiteMembers.java
index 5b03fd7652..99f1cd0531 100644
--- a/source/test-java/org/alfresco/rest/api/tests/TestSiteMembers.java
+++ b/source/test-java/org/alfresco/rest/api/tests/TestSiteMembers.java
@@ -500,6 +500,18 @@ public class TestSiteMembers extends EnterpriseTestApi
 				{
 					assertEquals(HttpStatus.SC_BAD_REQUEST, e.getHttpResponse().getStatusCode());
 				}
+				
+				// user is not a member of the site - 400
+				try
+				{
+					publicApiClient.setRequestContext(new RequestContext(network1.getId(), person2.getId()));
+					sitesProxy.updateSiteMember(site.getSiteId(), new SiteMember(person1.getId(), SiteRole.SiteContributor.toString()));
+					fail();
+				}
+				catch(PublicApiException e)
+				{
+					assertEquals(HttpStatus.SC_BAD_REQUEST, e.getHttpResponse().getStatusCode());
+				}
 		
 				// successful update
 				{