diff --git a/ctl/string.cc b/ctl/string.cc
index ff864c306..2a2b22eb1 100644
--- a/ctl/string.cc
+++ b/ctl/string.cc
@@ -99,7 +99,8 @@ string::reserve(size_t c2) noexcept
     if (!isbig()) {
         if (!(p2 = (char*)malloc(c2)))
             __builtin_trap();
-        memcpy(p2, data(), size() + 1);
+        memcpy(p2, data(), size());
+        p2[size()] = 0;
     } else {
         if (!(p2 = (char*)realloc(big()->p, c2)))
             __builtin_trap();
@@ -134,18 +135,18 @@ string::append(char ch) noexcept
     if (ckd_add(&n2, size(), 2))
         __builtin_trap();
     if (n2 > capacity()) {
-        size_t c2 = capacity() + 2;
+        size_t c2 = capacity();
         if (ckd_add(&c2, c2, c2 >> 1))
             __builtin_trap();
         reserve(c2);
     }
     data()[size()] = ch;
-    data()[size() + 1] = 0;
     if (isbig()) {
         ++big()->n;
     } else {
         --small()->rem;
     }
+    data()[size()] = 0;
 }
 
 void
diff --git a/ctl/string.h b/ctl/string.h
index 858b7a224..c830fdf12 100644
--- a/ctl/string.h
+++ b/ctl/string.h
@@ -151,7 +151,7 @@ class string
         if (isbig() && big()->c <= __::sso_max)
             __builtin_trap();
 #endif
-        return isbig() ? __::big_mask & big()->c : __::sso_max;
+        return isbig() ? __::big_mask & big()->c : __::string_size;
     }
 
     iterator begin() noexcept
diff --git a/test/ctl/string_test.cc b/test/ctl/string_test.cc
index 1a35cfda3..81464b571 100644
--- a/test/ctl/string_test.cc
+++ b/test/ctl/string_test.cc
@@ -18,6 +18,7 @@
 
 #include "ctl/string.h"
 
+#include <__type_traits/is_same.h>
 #include <__utility/move.h>
 
 #include "libc/runtime/runtime.h"
@@ -312,7 +313,7 @@ main()
             s.append(" world");
         }
         if (s != "hello world world world world world world world world world "
-            "world world") {
+                 "world world") {
             return 64;
         }
     }
@@ -353,6 +354,33 @@ main()
             return 78;
     }
 
+    {
+        ctl::string s;
+#undef ctl
+        if constexpr (std::is_same_v<ctl::string, decltype(s)>) {
+            // tests the small-string optimization on ctl::string
+            char* d = s.data();
+            for (int i = 0; i < 23; ++i) {
+                s.append("a");
+                if (s.data() != d) {
+                    return 79 + i;
+                }
+            }
+            s.append("a");
+            if (s.data() == d) {
+                return 103;
+            }
+        } else {
+            // just check that append in a loop works
+            for (int i = 0; i < 24; ++i) {
+                s.append("a");
+            }
+        }
+        if (s != "aaaaaaaaaaaaaaaaaaaaaaaa") {
+            return 104;
+        }
+    }
+
     CheckForMemoryLeaks();
     return 0;
 }