diff --git a/src/prelude/str.ts b/src/prelude/str.ts
index 5aede56..ec688ae 100644
--- a/src/prelude/str.ts
+++ b/src/prelude/str.ts
@@ -1,37 +1,37 @@
 // strings
 export const str = {
   join: (arr: string[], sep: string = ',') => arr.join(sep),
-  split: (str: string, sep: string = ',') => str.split(sep),
-  'to-upper': (str: string) => str.toUpperCase(),
-  'to-lower': (str: string) => str.toLowerCase(),
-  trim: (str: string) => str.trim(),
+  split: (str: string, sep: string = ',') => String(str ?? '').split(sep),
+  'to-upper': (str: string) => String(str ?? '').toUpperCase(),
+  'to-lower': (str: string) => String(str ?? '').toLowerCase(),
+  trim: (str: string) => String(str ?? '').trim(),
 
   // predicates
-  'starts-with?': (str: string, prefix: string) => str.startsWith(prefix),
-  'ends-with?': (str: string, suffix: string) => str.endsWith(suffix),
-  'contains?': (str: string, substr: string) => str.includes(substr),
-  'empty?': (str: string) => str.length === 0,
+  'starts-with?': (str: string, prefix: string) => String(str ?? '').startsWith(prefix),
+  'ends-with?': (str: string, suffix: string) => String(str ?? '').endsWith(suffix),
+  'contains?': (str: string, substr: string) => String(str ?? '').includes(substr),
+  'empty?': (str: string) => String(str ?? '').length === 0,
 
   // inspection
-  'index-of': (str: string, search: string) => str.indexOf(search),
-  'last-index-of': (str: string, search: string) => str.lastIndexOf(search),
+  'index-of': (str: string, search: string) => String(str ?? '').indexOf(search),
+  'last-index-of': (str: string, search: string) => String(str ?? '').lastIndexOf(search),
 
   // transformations
-  replace: (str: string, search: string, replacement: string) => str.replace(search, replacement),
-  'replace-all': (str: string, search: string, replacement: string) => str.replaceAll(search, replacement),
-  slice: (str: string, start: number, end?: number | null) => str.slice(start, end ?? undefined),
-  substring: (str: string, start: number, end?: number | null) => str.substring(start, end ?? undefined),
+  replace: (str: string, search: string, replacement: string) => String(str ?? '').replace(search, replacement),
+  'replace-all': (str: string, search: string, replacement: string) => String(str ?? '').replaceAll(search, replacement),
+  slice: (str: string, start: number, end?: number | null) => String(str ?? '').slice(start, end ?? undefined),
+  substring: (str: string, start: number, end?: number | null) => String(str ?? '').substring(start, end ?? undefined),
   repeat: (str: string, count: number) => {
     if (count < 0) throw new Error(`repeat: count must be non-negative, got ${count}`)
     if (!Number.isInteger(count)) throw new Error(`repeat: count must be an integer, got ${count}`)
-    return str.repeat(count)
+    return String(str ?? '').repeat(count)
   },
-  'pad-start': (str: string, length: number, pad: string = ' ') => str.padStart(length, pad),
-  'pad-end': (str: string, length: number, pad: string = ' ') => str.padEnd(length, pad),
-  lines: (str: string) => str.split('\n'),
-  chars: (str: string) => str.split(''),
+  'pad-start': (str: string, length: number, pad: string = ' ') => String(str ?? '').padStart(length, pad),
+  'pad-end': (str: string, length: number, pad: string = ' ') => String(str ?? '').padEnd(length, pad),
+  lines: (str: string) => String(str ?? '').split('\n'),
+  chars: (str: string) => String(str ?? '').split(''),
 
   // regex
-  match: (str: string, regex: RegExp) => str.match(regex),
-  'test?': (str: string, regex: RegExp) => regex.test(str),
+  match: (str: string, regex: RegExp) => String(str ?? '').match(regex),
+  'test?': (str: string, regex: RegExp) => regex.test(String(str ?? '')),
 }
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