diff --git a/src/mathed/InsetMathFrac.cpp b/src/mathed/InsetMathFrac.cpp
index 7388b7847b..43ae28d7c2 100644
--- a/src/mathed/InsetMathFrac.cpp
+++ b/src/mathed/InsetMathFrac.cpp
@@ -42,7 +42,9 @@ InsetMathFracBase::InsetMathFracBase(Buffer * buf, idx_type ncells)
 
 bool InsetMathFracBase::idxUpDown(Cursor & cur, bool up) const
 {
-	InsetMath::idx_type target = !up; // up ? 0 : 1, since upper cell has idx 0
+	// If we only have one cell, target = 0, otherwise
+	// target = up ? 0 : 1, since upper cell has idx 0
+	InsetMath::idx_type target = nargs() > 1 ? !up : 0;
 	if (cur.idx() == target)
 		return false;
 	cur.idx() = target;
diff --git a/status.16x b/status.16x
index 510a497288..831ab81f5e 100644
--- a/status.16x
+++ b/status.16x
@@ -129,6 +129,8 @@ What's new
 
 - Fix a crash when loading a document that specifies mutltiple labels
   for an equation (bug 5968).
+
+- Fix crash when moving down with the cursor in a Unit inset (bug 6351).
   
 - When using the PDF-Pages external inset, copy the included PDF file to
   the temporary directory, where LaTeX searches it (bug 6345).