diff --git a/app/src/lib/validation/variable-schema.ts b/app/src/lib/validation/variable-schema.ts
index ab1e5be..411e940 100644
--- a/app/src/lib/validation/variable-schema.ts
+++ b/app/src/lib/validation/variable-schema.ts
@@ -7,7 +7,12 @@ function fieldSchema(variable: VariableConfig): z.ZodTypeAny {
   switch (variable.type) {
     case "string": {
       let s = z.string();
-      if (variable.minLength !== undefined) s = s.min(variable.minLength);
+      // `required` alone has to reject "" even when the author didn't set an explicit
+      // minLength - otherwise an empty string satisfies a bare z.string() and the run starts
+      // with a "required" field effectively unset.
+      const minLength =
+        variable.minLength ?? (variable.required ? 1 : undefined);
+      if (minLength !== undefined) s = s.min(minLength);
       if (variable.maxLength !== undefined) s = s.max(variable.maxLength);
       if (variable.pattern) s = s.regex(new RegExp(variable.pattern));
       field = s;
@@ -26,9 +31,12 @@ function fieldSchema(variable: VariableConfig): z.ZodTypeAny {
     case "enum":
       field = z.enum(variable.choices as [string, ...string[]]);
       break;
-    case "multiselect":
-      field = z.array(z.enum(variable.choices as [string, ...string[]]));
+    case "multiselect": {
+      let arr = z.array(z.enum(variable.choices as [string, ...string[]]));
+      if (variable.required) arr = arr.min(1);
+      field = arr;
       break;
+    }
   }
 
   if (!variable.required) {