diff --git a/private/presenter.nim b/private/presenter.nim
index 1a709df..1a3c06a 100644
--- a/private/presenter.nim
+++ b/private/presenter.nim
@@ -253,7 +253,7 @@ proc present*(s: YamlStream, target: Stream, tagLib: YamlTagLibrary,
                 nextState = if length <= 60: dFlowSequenceStart else:
                             dBlockSequenceItem
             of ypsJson:
-                if levels[levels.high] in
+                if levels.len > 0 and levels[levels.high] in
                         [dFlowImplicitMapStart, dFlowImplicitMapValue]:
                     raise newException(YamlPresenterJsonError,
                             "Cannot have sequence as map key in JSON output!")
@@ -457,4 +457,4 @@ proc transform*(input: Stream, output: Stream, style: YamlPresentationStyle,
         tagLib = extendedTagLibrary()
         parser = newParser(tagLib)
     present(parser.parse(input), output, tagLib, style,
-            indentationStep)
\ No newline at end of file
+            indentationStep)