diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 787039307..97e1a6584 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -81,7 +81,9 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
     AttrPath::iterator i;
     // All attrpaths have at least one attr
     assert(!attrPath.empty());
-    for (i = attrPath.begin(); i + 1 < attrPath.end(); i++) {
+    // Checking attrPath validity.
+    // ===========================
+    for (i = attrPath.begin(); i + 1 < attrPath.end(); i++) { 
         if (i->symbol.set()) {
             ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
             if (j != attrs->attrs.end()) {
@@ -102,11 +104,29 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             attrs = nested;
         }
     }
-    if (i->symbol.set()) {
+    // Expr insertion.
+    // ==========================
+    if (i->symbol.set()) { 
         ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
         if (j != attrs->attrs.end()) {
-            dupAttr(attrPath, pos, j->second.pos);
+            // This attr path is already defined. However, if both
+            // e and the expr pointed by the attr path are two attribute sets,
+            // we want to merge them.
+            // Otherwise, throw an error.
+            ExprAttrs* ae = dynamic_cast<ExprAttrs *>(e);
+            ExprAttrs* jAttrs = dynamic_cast<ExprAttrs *>(j->second.e);
+            if (jAttrs && ae) {
+                for (auto ad: ae->attrs) {
+                    ExprAttrs::AttrDefs::iterator j2 = jAttrs->attrs.find(ad.first);
+                    if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
+                        dupAttr(ad.first, j2->second.pos, ad.second.pos);
+                    jAttrs->attrs[ad.first] = ad.second;
+                }
+            } else {
+                dupAttr(attrPath, pos, j->second.pos);
+            }
         } else {
+            // This attr path is not defined. Let's create it.
             attrs->attrs[i->symbol] = ExprAttrs::AttrDef(e, pos);
             e->setName(i->symbol);
         }
diff --git a/tests/lang/parse-fail-mixed-nested-attrs1.nix b/tests/lang/parse-fail-mixed-nested-attrs1.nix
new file mode 100644
index 000000000..11e40e66f
--- /dev/null
+++ b/tests/lang/parse-fail-mixed-nested-attrs1.nix
@@ -0,0 +1,4 @@
+{ 
+  x.z = 3; 
+  x = { y = 3; z = 3; }; 
+}
diff --git a/tests/lang/parse-fail-mixed-nested-attrs2.nix b/tests/lang/parse-fail-mixed-nested-attrs2.nix
new file mode 100644
index 000000000..17da82e5f
--- /dev/null
+++ b/tests/lang/parse-fail-mixed-nested-attrs2.nix
@@ -0,0 +1,4 @@
+{ 
+  x.y.y = 3; 
+  x = { y.y= 3; z = 3; }; 
+}
diff --git a/tests/lang/parse-fail-dup-attrs-6.nix b/tests/lang/parse-okay-dup-attrs-6.nix
similarity index 100%
rename from tests/lang/parse-fail-dup-attrs-6.nix
rename to tests/lang/parse-okay-dup-attrs-6.nix
diff --git a/tests/lang/parse-okay-mixed-nested-attrs-1.nix b/tests/lang/parse-okay-mixed-nested-attrs-1.nix
new file mode 100644
index 000000000..fd1001c8c
--- /dev/null
+++ b/tests/lang/parse-okay-mixed-nested-attrs-1.nix
@@ -0,0 +1,4 @@
+{ 
+  x = { y = 3; z = 3; }; 
+  x.q = 3; 
+}
diff --git a/tests/lang/parse-okay-mixed-nested-attrs-2.nix b/tests/lang/parse-okay-mixed-nested-attrs-2.nix
new file mode 100644
index 000000000..ad066b680
--- /dev/null
+++ b/tests/lang/parse-okay-mixed-nested-attrs-2.nix
@@ -0,0 +1,4 @@
+{ 
+  x.q = 3; 
+  x = { y = 3; z = 3; }; 
+}
diff --git a/tests/lang/parse-okay-mixed-nested-attrs-3.nix b/tests/lang/parse-okay-mixed-nested-attrs-3.nix
new file mode 100644
index 000000000..45a33e480
--- /dev/null
+++ b/tests/lang/parse-okay-mixed-nested-attrs-3.nix
@@ -0,0 +1,7 @@
+{
+    services.ssh.enable = true;
+    services.ssh = { port = 123; };
+    services = {
+        httpd.enable = true;
+    };
+}