diff --git a/common/predictive-text/worker/model-compositor.ts b/common/predictive-text/worker/model-compositor.ts
index 3b05b9ad28..97199b3383 100644
--- a/common/predictive-text/worker/model-compositor.ts
+++ b/common/predictive-text/worker/model-compositor.ts
@@ -487,7 +487,11 @@ class ModelCompositor {
     let postContextTokenization = this.tokenize(postContext);
     if(postContextTokenization) {
       // Handles display string for reversions triggered by accepting a suggestion mid-token.
-      revertedPrefix = postContextTokenization.left[postContextTokenization.left.length-1];
+      if(postContextTokenization.left.length > 0) {
+        revertedPrefix = postContextTokenization.left[postContextTokenization.left.length-1];
+      } else {
+        revertedPrefix = '';
+      }
       revertedPrefix += postContextTokenization.caretSplitsToken ? postContextTokenization.right[0] : '';
     } else {
       revertedPrefix = this.wordbreak(postContext);