diff --git a/lib/system.nim b/lib/system.nim
index a7b22a2839..e8b427c554 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -4028,6 +4028,8 @@ proc shallow*(s: var string) {.noSideEffect, inline.} =
   ## purposes.
   when not defined(JS) and not defined(nimscript) and not defined(gcDestructors):
     var s = cast[PGenericSeq](s)
+    if s == nil:
+      s = cast[PGenericSeq](newString(0))
     # string literals cannot become 'shallow':
     if (s.reserved and strlitFlag) == 0:
       s.reserved = s.reserved or seqShallowFlag
diff --git a/tests/system/tnilconcats.nim b/tests/system/tnilconcats.nim
index 5e4a1b3176..c1126405c5 100644
--- a/tests/system/tnilconcats.nim
+++ b/tests/system/tnilconcats.nim
@@ -23,3 +23,10 @@ when true:
   doAssert s == "fooabc"
 
   echo x
+
+  # casting an empty string as sequence with shallow() should not segfault
+  var s2: string
+  shallow(s2)
+  s2 &= "foo"
+  doAssert s2 == "foo"
+