diff --git a/core/themes/bartik/css/components/form.css b/core/themes/bartik/css/components/form.css
index dd5ac0c..a8fcdd6 100644
--- a/core/themes/bartik/css/components/form.css
+++ b/core/themes/bartik/css/components/form.css
@@ -15,6 +15,7 @@ form {
 }
 fieldset {
   margin: 1em 0;
+  min-width: 0;
 }
 details,
 fieldset,
diff --git a/core/themes/seven/css/components/form.css b/core/themes/seven/css/components/form.css
index f74eb74..c1a16ab 100644
--- a/core/themes/seven/css/components/form.css
+++ b/core/themes/seven/css/components/form.css
@@ -10,6 +10,7 @@ fieldset:not(.fieldgroup) {
   border-radius: 2px;
   margin: 1em 0;
   padding: 30px 18px 18px;
+  min-width: 0;
   position: relative;
 }
 fieldset:not(.fieldgroup) > legend {