diff --git a/lightstep/http_converter.py b/lightstep/http_converter.py
index c17573d..64f9b1b 100644
--- a/lightstep/http_converter.py
+++ b/lightstep/http_converter.py
@@ -40,6 +40,8 @@ def create_span_record(self, span, guid):
         span_context = SpanContext(trace_id=span.context.trace_id,
                                    span_id=span.context.span_id)
         seconds, nanos = util._time_to_seconds_nanos(span.start_time)
+        if span.duration < 0:
+            span.duration = 0
         span_record = Span(span_context=span_context,
                            operation_name=util._coerce_str(span.operation_name),
                            start_timestamp=Timestamp(seconds=seconds, nanos=nanos),