diff --git a/lib/philomena/search/parser.ex b/lib/philomena/search/parser.ex
index 00bbed6a..f89290ee 100644
--- a/lib/philomena/search/parser.ex
+++ b/lib/philomena/search/parser.ex
@@ -107,9 +107,12 @@ defmodule Philomena.Search.Parser do
 
   defp search_boost(parser, tokens) do
     case search_not(parser, tokens) do
-      {:ok, {child, [{:boost, value} | r_tokens]}} ->
+      {:ok, {child, [{:boost, value} | r_tokens]}} when value >= 0 ->
         {:ok, {%{function_score: %{query: child, boost: value}}, r_tokens}}
 
+      {:ok, {_child, [{:boost, _value} | _r_tokens]}} ->
+        {:error, "Boost value must be non-negative."}
+
       value ->
         value
     end