diff --git a/interface-spec.html b/interface-spec.html
index f0f7b01..5f4cc36 100644
--- a/interface-spec.html
+++ b/interface-spec.html
@@ -129,7 +129,7 @@ <h3><dfn>Term</dfn> interface</h3>
     interface Term {
       attribute string termType;
       attribute string value;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -168,7 +168,7 @@ <h3><dfn>NamedNode</dfn> interface</h3>
     interface NamedNode : Term {
       attribute string termType;
       attribute string value;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -193,7 +193,7 @@ <h3><dfn>BlankNode</dfn> interface</h3>
     interface BlankNode : Term {
       attribute string termType;
       attribute string value;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -222,7 +222,7 @@ <h3><dfn>Literal</dfn> interface</h3>
       attribute string value;
       attribute string language;
       attribute NamedNode datatype;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -263,7 +263,7 @@ <h3><dfn>Variable</dfn> interface</h3>
     interface Variable : Term {
       attribute string termType;
       attribute string value;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -289,7 +289,7 @@ <h3><dfn>DefaultGraph</dfn> interface</h3>
     interface DefaultGraph : Term {
       attribute string termType;
       attribute string value;
-      boolean equals(Term other);
+      boolean equals(optional Term? other);
     };
     </pre>
 
@@ -333,7 +333,7 @@ <h3><dfn>Quad</dfn> interface</h3>
       attribute Term predicate;
       attribute Term object;
       attribute Term graph;
-      boolean equals(Quad other);
+      boolean equals(optional Quad? other);
     };
     </pre>