diff --git a/pkgs/ruby/ext/default/ntoa.c b/pkgs/ruby/ext/default/ntoa.c
index 668ec95..7eebe3e 100644
--- a/pkgs/ruby/ext/default/ntoa.c
+++ b/pkgs/ruby/ext/default/ntoa.c
@@ -4,10 +4,10 @@
 #define MAX_PREFIX_LEN 100
 
 #ifdef HAVE_KERNAUX_UTOA
-static VALUE rb_KernAux_utoa(VALUE self, VALUE number, VALUE base, VALUE prefix);
+static VALUE rb_KernAux_utoa(int argc, const VALUE *argv, VALUE self);
 #endif
 #ifdef HAVE_KERNAUX_ITOA
-static VALUE rb_KernAux_itoa(VALUE self, VALUE number, VALUE base, VALUE prefix);
+static VALUE rb_KernAux_itoa(int argc, const VALUE *argv, VALUE self);
 #endif
 #ifdef HAVE_KERNAUX_UTOA10
 static VALUE rb_KernAux_utoa10(VALUE self, VALUE number);
@@ -70,10 +70,10 @@ void init_ntoa()
                               rb_KernAux_Error));
 
 #ifdef HAVE_KERNAUX_UTOA
-    rb_define_singleton_method(rb_KernAux, "utoa", rb_KernAux_utoa, 3);
+    rb_define_singleton_method(rb_KernAux, "utoa", rb_KernAux_utoa, -1);
 #endif
 #ifdef HAVE_KERNAUX_ITOA
-    rb_define_singleton_method(rb_KernAux, "itoa", rb_KernAux_itoa, 3);
+    rb_define_singleton_method(rb_KernAux, "itoa", rb_KernAux_itoa, -1);
 #endif
 #ifdef HAVE_KERNAUX_UTOA10
     rb_define_singleton_method(rb_KernAux, "utoa10", rb_KernAux_utoa10, 1);
@@ -90,12 +90,20 @@ void init_ntoa()
 }
 
 #ifdef HAVE_KERNAUX_UTOA
-VALUE rb_KernAux_utoa(
-    const VALUE self_rb __attribute__((unused)),
-    const VALUE number_rb,
-    const VALUE base_rb,
-    VALUE prefix_rb
-) {
+VALUE rb_KernAux_utoa(const int argc, const VALUE *argv, const VALUE self)
+{
+    if (argc < 2 || argc > 3) {
+        rb_raise(
+            rb_eArgError,
+            "wrong number of arguments (given %d, expected 2..3)",
+            argc
+        );
+    }
+
+    VALUE number_rb = argv[0];
+    VALUE base_rb   = argv[1];
+    VALUE prefix_rb = argc == 3 ? argv[2] : Qnil;
+
     RB_INTEGER_TYPE_P(number_rb);
     if (rb_funcall(number_rb, rb_intern_LESS, 1, INT2FIX(0))) {
         rb_raise(rb_eRangeError, "can't convert negative number to uint64_t");
@@ -123,12 +131,20 @@ VALUE rb_KernAux_utoa(
 #endif
 
 #ifdef HAVE_KERNAUX_ITOA
-VALUE rb_KernAux_itoa(
-    const VALUE self_rb __attribute__((unused)),
-    const VALUE number_rb,
-    const VALUE base_rb,
-    VALUE prefix_rb
-) {
+VALUE rb_KernAux_itoa(const int argc, const VALUE *argv, const VALUE self)
+{
+    if (argc < 2 || argc > 3) {
+        rb_raise(
+            rb_eArgError,
+            "wrong number of arguments (given %d, expected 2..3)",
+            argc
+        );
+    }
+
+    VALUE number_rb = argv[0];
+    VALUE base_rb   = argv[1];
+    VALUE prefix_rb = argc == 3 ? argv[2] : Qnil;
+
     RB_INTEGER_TYPE_P(number_rb);
 
     const char *prefix = NULL;
diff --git a/pkgs/ruby/spec/lib/kernaux/itoa_spec.rb b/pkgs/ruby/spec/lib/kernaux/itoa_spec.rb
index 2c3f618..d5c891f 100644
--- a/pkgs/ruby/spec/lib/kernaux/itoa_spec.rb
+++ b/pkgs/ruby/spec/lib/kernaux/itoa_spec.rb
@@ -169,6 +169,14 @@ RSpec.describe KernAux, '.itoa' do
       end
     end
 
+    context 'when no prefix is given' do
+      subject(:itoa) { described_class.itoa number, base }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq number.to_s base }
+    end
+
     context 'when prefix is nil' do
       let(:prefix) { nil }
 
diff --git a/pkgs/ruby/spec/lib/kernaux/utoa_spec.rb b/pkgs/ruby/spec/lib/kernaux/utoa_spec.rb
index ed46369..82eedbf 100644
--- a/pkgs/ruby/spec/lib/kernaux/utoa_spec.rb
+++ b/pkgs/ruby/spec/lib/kernaux/utoa_spec.rb
@@ -145,6 +145,14 @@ RSpec.describe KernAux, '.utoa' do
       end
     end
 
+    context 'when no prefix is given' do
+      subject(:utoa) { described_class.utoa number, base }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq number.to_s base }
+    end
+
     context 'when prefix is nil' do
       let(:prefix) { nil }