diff --git a/lib/coffee-script/nodes.js b/lib/coffee-script/nodes.js
index d36a5465..8ff9b99c 100644
--- a/lib/coffee-script/nodes.js
+++ b/lib/coffee-script/nodes.js
@@ -1501,10 +1501,13 @@
     };
 
     Assign.prototype.compileConditional = function(o) {
-      var left, rite, _ref3;
-      _ref3 = this.variable.cacheReference(o), left = _ref3[0], rite = _ref3[1];
+      var left, right, _ref3;
+      _ref3 = this.variable.cacheReference(o), left = _ref3[0], right = _ref3[1];
+      if (left.base instanceof Literal && left.base.value !== "this" && !o.scope.check(left.base.value)) {
+        throw new Error("the variable \"" + left.base.value + "\" can't be assigned with " + this.context + " because it has not been defined.");
+      }
       if (__indexOf.call(this.context, "?") >= 0) o.isExistentialEquals = true;
-      return new Op(this.context.slice(0, -1), left, new Assign(rite, this.value, '=')).compile(o);
+      return new Op(this.context.slice(0, -1), left, new Assign(right, this.value, '=')).compile(o);
     };
 
     Assign.prototype.compileSplice = function(o) {
diff --git a/src/nodes.coffee b/src/nodes.coffee
index b19f35b1..6aee1e96 100644
--- a/src/nodes.coffee
+++ b/src/nodes.coffee
@@ -1117,9 +1117,12 @@ exports.Assign = class Assign extends Base
   # operands are only evaluated once, even though we have to reference them
   # more than once.
   compileConditional: (o) ->
-    [left, rite] = @variable.cacheReference o
+    [left, right] = @variable.cacheReference o
+    # Disallow conditional assignment of undefined variables.
+    if left.base instanceof Literal and left.base.value != "this" and not o.scope.check left.base.value
+      throw new Error "the variable \"#{left.base.value}\" can't be assigned with #{@context} because it has not been defined."
     if "?" in @context then o.isExistentialEquals = true
-    new Op(@context[...-1], left, new Assign(rite, @value, '=') ).compile o
+    new Op(@context[...-1], left, new Assign(right, @value, '=') ).compile o
 
   # Compile the assignment from an array splice literal, using JavaScript's
   # `Array#splice` method.
diff --git a/test/assignment.coffee b/test/assignment.coffee
index 3e8b9783..dd4bbb0c 100644
--- a/test/assignment.coffee
+++ b/test/assignment.coffee
@@ -19,12 +19,6 @@ test "unassignable values", ->
   for nonref in ['', '""', '0', 'f()'].concat CoffeeScript.RESERVED
     eq nonce, (try CoffeeScript.compile "#{nonref} = v" catch e then nonce)
 
-test "compound assignments should not declare", ->
-  # TODO: make description more clear
-  # TODO: remove reference to Math
-  eq Math, (-> Math or= 0)()
-
-
 # Compound Assignment
 
 test "boolean operators", ->
@@ -285,8 +279,23 @@ test "existential assignment", ->
   c = null
   c ?= nonce
   eq nonce, c
-  d ?= nonce
-  eq nonce, d
+
+test "#1627: prohibit conditional assignment of undefined variables", ->
+  throws (-> CoffeeScript.compile "x ?= 10"),        null, "prohibit (x ?= 10)"
+  throws (-> CoffeeScript.compile "x ||= 10"),       null, "prohibit (x ||= 10)"
+  throws (-> CoffeeScript.compile "x or= 10"),       null, "prohibit (x or= 10)"
+  throws (-> CoffeeScript.compile "do -> x ?= 10"),  null, "prohibit (do -> x ?= 10)"
+  throws (-> CoffeeScript.compile "do -> x ||= 10"), null, "prohibit (do -> x ||= 10)"
+  throws (-> CoffeeScript.compile "do -> x or= 10"), null, "prohibit (do -> x or= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; x ?= 10"),        "allow (x = null; x ?= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; x ||= 10"),       "allow (x = null; x ||= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; x or= 10"),       "allow (x = null; x or= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; do -> x ?= 10"),  "allow (x = null; do -> x ?= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; do -> x ||= 10"), "allow (x = null; do -> x ||= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; do -> x or= 10"), "allow (x = null; do -> x or= 10)"
+  
+  throws (-> CoffeeScript.compile "-> -> -> x ?= 10"), null, "prohibit (-> -> -> x ?= 10)"
+  doesNotThrow (-> CoffeeScript.compile "x = null; -> -> -> x ?= 10"), "allow (x = null; -> -> -> x ?= 10)"
 
 test "#1348, #1216: existential assignment compilation", ->
   nonce = {}
@@ -295,14 +304,7 @@ test "#1348, #1216: existential assignment compilation", ->
   eq nonce, b
   #the first ?= compiles into a statement; the second ?= compiles to a ternary expression
   eq a ?= b ?= 1, nonce
-
-  e ?= f ?= g ?= 1
-  eq e + g, 2
-
-  #need to ensure the two vars are not defined, hence the strange names;
-  # broke earlier when using c ?= d ?= 1 because `d` is declared elsewhere
-  eq und1_1348 ?= und2_1348 ?= 1, 1
-
+  
   if a then a ?= 2 else a = 3
   eq a, nonce