diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 2ea13d52010d..6bb4bd2c0c62 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -321,6 +321,7 @@ describe "String" do
     it { expect_raises(ArgumentError) { "1__234".to_i } }
     it { expect_raises(ArgumentError) { "1_234".to_i } }
     it { expect_raises(ArgumentError) { "   1234   ".to_i(whitespace: false) } }
+    it { expect_raises(ArgumentError) { "".to_i(whitespace: false) } }
     it { expect_raises(ArgumentError) { "0x123".to_i } }
     it { expect_raises(ArgumentError) { "0b123".to_i } }
     it { expect_raises(ArgumentError) { "000b123".to_i(prefix: true) } }
@@ -515,6 +516,7 @@ describe "String" do
     "nan".to_f?(whitespace: false).try(&.nan?).should be_true
     " nan".to_f?(whitespace: false).should be_nil
     "nan ".to_f?(whitespace: false).should be_nil
+    expect_raises(ArgumentError) { "".to_f(whitespace: false) }
     "nani".to_f?(strict: true).should be_nil
     " INF".to_f?.should eq Float64::INFINITY
     "INF".to_f?.should eq Float64::INFINITY
diff --git a/src/string.cr b/src/string.cr
index 08bbb87fc505..cf96401253b8 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -752,7 +752,8 @@ class String
   end
 
   private def to_f_impl(whitespace : Bool = true, strict : Bool = true, &)
-    return unless whitespace || '0' <= self[0] <= '9' || self[0].in?('-', '+', 'i', 'I', 'n', 'N')
+    return unless first_char = self[0]?
+    return unless whitespace || '0' <= first_char <= '9' || first_char.in?('-', '+', 'i', 'I', 'n', 'N')
 
     v, endptr = yield