diff --git a/M2/Macaulay2/m2/mutablemat.m2 b/M2/Macaulay2/m2/mutablemat.m2
index ab0f0169c5..29fa740483 100644
--- a/M2/Macaulay2/m2/mutablemat.m2
+++ b/M2/Macaulay2/m2/mutablemat.m2
@@ -16,6 +16,7 @@ precision MutableMatrix := precision @@ ring
 expression MutableMatrix := m -> MatrixExpression append(applyTable(entries m, expression), symbol MutableMatrix => true)
 texMath MutableMatrix := m -> texMath expression m
 net MutableMatrix := m -> net expression m
+toExternalString MutableMatrix := lookup(toExternalString, MutableHashTable)
 
 map(Ring,RawMutableMatrix) := opts -> (R,m) -> (
      new MutableMatrix from {