diff --git a/changelog.md b/changelog.md
index 2e1795bab5..6cd0faf528 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,4 +1,4 @@
-## v0.X.X - XX/XX/2018
+## v0.19.X - XX/XX/2018
 
 ### Changes affecting backwards compatibility
 
@@ -14,6 +14,9 @@
 - Assignments that would "slice" an object into its supertype are now prevented
   at runtime. Use ``ref object`` with inheritance rather than ``object`` with
   inheritance to prevent this issue.
+- The ``not nil`` type annotation now has to be enabled explicitly
+  via ``{.experimental: "notnil"}`` as we are still not pleased with how this
+  feature works with Nim's containers.
 
 
 #### Breaking changes in the standard library
diff --git a/compiler/condsyms.nim b/compiler/condsyms.nim
index b1d0ccc7a8..f8a75e68e3 100644
--- a/compiler/condsyms.nim
+++ b/compiler/condsyms.nim
@@ -116,3 +116,4 @@ proc initDefines*() =
   defineSymbol("nimVmEqIdent")
   defineSymbol("nimNoNil")
   defineSymbol("nimNoZeroTerminator")
+  defineSymbol("nimNotNil")
diff --git a/compiler/options.nim b/compiler/options.nim
index 9f56d3f180..5baaa1bfd5 100644
--- a/compiler/options.nim
+++ b/compiler/options.nim
@@ -109,7 +109,8 @@ type
     dotOperators,
     callOperator,
     parallel,
-    destructor
+    destructor,
+    notnil
 
   ConfigRef* = ref object ## eventually all global configuration should be moved here
     cppDefines*: HashSet[string]
diff --git a/compiler/semtypes.nim b/compiler/semtypes.nim
index 1fc263617b..537319d66f 100644
--- a/compiler/semtypes.nim
+++ b/compiler/semtypes.nim
@@ -1389,6 +1389,8 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
               n.sons[2].kind == nkNilLit:
             result = freshType(result, prev)
             result.flags.incl(tfNotNil)
+            if notnil notin c.features:
+              localError(n.info, "enable the 'not nil' annotation with {.experimental: \"notnil\".}")
           else:
             localError(n.info, errGenerated, "invalid type")
         of 2:
diff --git a/lib/system.nim b/lib/system.nim
index 5c0970f853..0c8659fdaf 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -3223,7 +3223,7 @@ when not defined(JS): #and not defined(nimscript):
     when declared(initGC): initGC()
 
   when not defined(nimscript):
-    proc setControlCHook*(hook: proc () {.noconv.} not nil)
+    proc setControlCHook*(hook: proc () {.noconv.})
       ## allows you to override the behaviour of your application when CTRL+C
       ## is pressed. Only one such hook is supported.
 
diff --git a/lib/system/excpt.nim b/lib/system/excpt.nim
index afeab2b6c3..fb38948f7b 100644
--- a/lib/system/excpt.nim
+++ b/lib/system/excpt.nim
@@ -56,7 +56,7 @@ var
     # list of exception handlers
     # a global variable for the root of all try blocks
   currException {.threadvar.}: ref Exception
-  raise_counter {.threadvar.}: uint 
+  raise_counter {.threadvar.}: uint
 
   gcFramePtr {.threadvar.}: GcFrame
 
@@ -126,10 +126,10 @@ proc popCurrentExceptionEx(id: uint) {.compilerRtl.} =
     while cur != nil and cur.raise_id != id:
       prev = cur
       cur = cur.up
-    if cur == nil: 
+    if cur == nil:
       showErrorMessage("popCurrentExceptionEx() exception was not found in the exception stack. Aborting...")
       quitOrDebug()
-    prev.up = cur.up  
+    prev.up = cur.up
 
 # some platforms have native support for stack traces:
 const
@@ -503,7 +503,7 @@ when not defined(noSignalHandler) and not defined(useNimRtl):
 
   registerSignalHandler() # call it in initialization section
 
-proc setControlCHook(hook: proc () {.noconv.} not nil) =
+proc setControlCHook(hook: proc () {.noconv.}) =
   # ugly cast, but should work on all architectures:
   type SignalHandler = proc (sign: cint) {.noconv, benign.}
   c_signal(SIGINT, cast[SignalHandler](hook))
diff --git a/tests/notnil/tmust_compile.nim b/tests/notnil/tmust_compile.nim
index 10da154f00..a32c6c7ecd 100644
--- a/tests/notnil/tmust_compile.nim
+++ b/tests/notnil/tmust_compile.nim
@@ -3,6 +3,7 @@ discard """
 """
 
 # bug #6682
+{.experimental: "notnil".}
 
 type
     Fields = enum
diff --git a/tests/notnil/tnotnil.nim b/tests/notnil/tnotnil.nim
index f65634ed66..e392b155c6 100644
--- a/tests/notnil/tnotnil.nim
+++ b/tests/notnil/tnotnil.nim
@@ -2,7 +2,7 @@ discard """
   line: 22
   errormsg: "type mismatch"
 """
-
+{.experimental: "notnil".}
 type
   PObj = ref TObj not nil
   TObj = object
@@ -15,8 +15,8 @@ type
 proc p(x: string not nil): int =
   result = 45
 
-proc q(x: MyString) = nil
-proc q2(x: string) = nil
+proc q(x: MyString) = discard
+proc q2(x: string) = discard
 
 q2(nil)
 q(nil)
diff --git a/tests/notnil/tnotnil1.nim b/tests/notnil/tnotnil1.nim
index 73472752c7..7f9d022958 100644
--- a/tests/notnil/tnotnil1.nim
+++ b/tests/notnil/tnotnil1.nim
@@ -4,7 +4,7 @@ discard """
 """
 
 import strutils
-
+{.experimental: "notnil".}
 
 type
   TObj = object
@@ -18,13 +18,13 @@ proc q(s: superstring) =
   echo s
 
 proc p2() =
-  var  a: string = "I am not nil"
+  var a: string = "I am not nil"
   q(a) # but this should and does not
 
 p2()
 
 proc q(x: pointer not nil) =
-  nil
+  discard
 
 proc p() =
   var x: pointer
diff --git a/tests/notnil/tnotnil2.nim b/tests/notnil/tnotnil2.nim
index bd6b8b6755..6ab7bf9519 100644
--- a/tests/notnil/tnotnil2.nim
+++ b/tests/notnil/tnotnil2.nim
@@ -4,7 +4,7 @@ discard """
 """
 
 import strutils
-
+{.experimental: "notnil".}
 
 type
   TObj = object
diff --git a/tests/notnil/tnotnil3.nim b/tests/notnil/tnotnil3.nim
index b7c7a811d1..31a4efef78 100644
--- a/tests/notnil/tnotnil3.nim
+++ b/tests/notnil/tnotnil3.nim
@@ -5,7 +5,7 @@ discard """
 
 # bug #584
 # Testprogram for 'not nil' check
-
+{.experimental: "notnil".}
 const testWithResult = true
 
 type
diff --git a/tests/notnil/tnotnil4.nim b/tests/notnil/tnotnil4.nim
index 2fa888357f..4fd169827d 100644
--- a/tests/notnil/tnotnil4.nim
+++ b/tests/notnil/tnotnil4.nim
@@ -2,6 +2,8 @@ discard ""
 type
    TObj = ref object
 
+{.experimental: "notnil".}
+
 proc check(a: TObj not nil) =
   echo repr(a)
 
diff --git a/tests/notnil/tnotnil_in_generic.nim b/tests/notnil/tnotnil_in_generic.nim
index 357ab2c7c7..89d20f1826 100644
--- a/tests/notnil/tnotnil_in_generic.nim
+++ b/tests/notnil/tnotnil_in_generic.nim
@@ -3,6 +3,7 @@ discard """
 """
 
 # bug #2216
+{.experimental: "notnil".}
 
 type
     A[T] = ref object
diff --git a/tests/notnil/tnotnil_in_objconstr.nim b/tests/notnil/tnotnil_in_objconstr.nim
index 7dce98c29f..d33709906b 100644
--- a/tests/notnil/tnotnil_in_objconstr.nim
+++ b/tests/notnil/tnotnil_in_objconstr.nim
@@ -2,7 +2,7 @@ discard """
   errormsg: "fields not initialized: bar"
   line: "13"
 """
-
+{.experimental: "notnil".}
 # bug #2355
 type
   Foo = object