diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 0e4f2519ac..9acd423108 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1019,9 +1019,17 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         case tBool:
             return v1.boolean == v2.boolean;
 
-        case tString:
-            /* !!! contexts */
-            return strcmp(v1.string.s, v2.string.s) == 0;
+        case tString: {
+            /* Compare both the string and its context. */
+            if (strcmp(v1.string.s, v2.string.s) != 0) return false;
+            const char * * p = v1.string.context, * * q = v2.string.context;
+            if (!p && !q) return true;
+            if (!p || !q) return false;
+            for ( ; *p && *q; ++p, ++q)
+                if (strcmp(*p, *q) != 0) return false;
+            if (*p || *q) return false;
+            return true;
+        }
 
         case tPath:
             return strcmp(v1.path, v2.path) == 0;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index aff68ea2ab..2726fd971d 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -69,7 +69,7 @@ struct Value
            For canonicity, the store paths should be in sorted order. */
         struct {
             const char * s;
-            const char * * context;
+            const char * * context; // must be in sorted order
         } string;
         
         const char * path;