diff --git a/src/NJsonApiCore/ResourceMapping.cs b/src/NJsonApiCore/ResourceMapping.cs
index 1c84d16..85af5b8 100644
--- a/src/NJsonApiCore/ResourceMapping.cs
+++ b/src/NJsonApiCore/ResourceMapping.cs
@@ -60,7 +60,7 @@ public bool ValidateIncludedRelationshipPaths(string[] includedPaths)
                 var parts = relationshipPath.Split('.');
                 foreach (var part in parts)
                 {
-                    var relationship = currentMapping.Relationships.SingleOrDefault(x => x.RelatedBaseResourceType == part);
+                    var relationship = currentMapping.Relationships.FirstOrDefault(x => x.RelatedBaseResourceType == part);
                     if (relationship == null)
                         return false;
 
@@ -93,4 +93,4 @@ public Dictionary<string, object> GetValuesFromAttributes(Dictionary<string, obj
             return values;
         }
     }
-}
\ No newline at end of file
+}