diff --git a/networks-tests.rkt b/networks-tests.rkt
index 1c1b427..dd7fdac 100644
--- a/networks-tests.rkt
+++ b/networks-tests.rkt
@@ -114,7 +114,16 @@
     (check-equal? (edge-weight sig3 'a 'a) '+)
     (check-equal? (edge-weight sig3 'b 'b) '+)
     (check-equal? (edge-weight sig3 'a 'b) '+)
-    (check-equal? (edge-weight sig3 'b 'a) '-)))
+    (check-equal? (edge-weight sig3 'b 'a) '-))
+  (let* ([n #hash((a . #t) (b . #t))]
+         [ig (build-interaction-graph n)]
+         [sig-nf (build-boolean-signed-interaction-graph/form n)]
+         [sig (build-boolean-signed-interaction-graph (network-form->network n))])
+    (check-equal? (get-vertices ig) '(b a))
+    (check-true (empty? (get-edges ig)))
+    (check-equal? (get-vertices sig-nf) '(b a))
+    (check-true (empty? (get-edges sig-nf)))
+    (check-equal? (get-vertices sig) '(b a))))
 
 (test-case "Dynamics of networks"
   (check-equal? (pretty-print-state (make-state '((a . #f) (b . 3) (c . 4)))) "a:#f b:3 c:4")
diff --git a/networks.rkt b/networks.rkt
index ce9401f..ee381b0 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -286,11 +286,18 @@
 (define (build-signed-interaction-graph/form network-form doms)
   (let ([ig (build-interaction-graph network-form)]
         [network (network-form->network network-form)])
-    (weighted-graph/directed
-     (for/list ([e (in-edges ig)])
-       (match-let ([(list x y) e])
-         (list (get-interaction-sign network doms x y)
-               x y))))))
+    ;; Label every edge of the interaction graph with the sign.
+    (define sig
+      (weighted-graph/directed
+       (for/list ([e (in-edges ig)])
+         (match-let ([(list x y) e])
+           (list (get-interaction-sign network doms x y)
+                 x y)))))
+    ;; Ensure that every variable of the network appears in the signed
+    ;; interaction graph as well.
+    (for ([v (in-vertices ig)])
+      (add-vertex! sig v))
+    sig))
 
 ;;; Calls build-signed-interaction-graph with the Boolean domain for
 ;;; all variable.
@@ -314,13 +321,19 @@
 ;;; every arrow in the unsigned interaction graph, so its performance
 ;;; decreases very quickly with the size of the network.
 (define (build-signed-interaction-graph network doms)
-  (weighted-graph/directed
-   (for*/fold ([edges '()])
-              ([(x _) (in-hash network)]
-               [(y _) (in-hash network)])
-     (match (get-interaction-sign network doms x y)
-       ['0 edges]
-       [sign (cons (list sign x y) edges)]))))
+  (define sig
+    (weighted-graph/directed
+     (for*/fold ([edges '()])
+                ([(x _) (in-hash network)]
+                 [(y _) (in-hash network)])
+       (match (get-interaction-sign network doms x y)
+         ['0 edges]
+         [sign (cons (list sign x y) edges)]))))
+  ;; Ensure that all variables of the network appear in the signed
+  ;; interaction graph.
+  (for ([(v _) (in-hash network)])
+      (add-vertex! sig v))
+    sig)
 
 ;;; Calls build-signed-interaction-graph assuming that the domains of
 ;;; all variables are Boolean.