diff --git a/src/FindFirstFunctions.jl b/src/FindFirstFunctions.jl
index c324303..1c7fc52 100644
--- a/src/FindFirstFunctions.jl
+++ b/src/FindFirstFunctions.jl
@@ -240,7 +240,15 @@ function (g::Guesser)(x)
             f > 0 ? lastindex(v) : firstindex(v)
         else
             i_0, i_f = firstindex(v), lastindex(v)
-            round(typeof(firstindex(v)), f * (i_f - i_0) + i_0)
+            i_approx = f * (i_f - i_0) + i_0
+            target_type = typeof(firstindex(v))
+            if i_approx >= typemax(target_type)
+                lastindex(v) + 1
+            elseif i_approx <= typemin(target_type)
+                firstindex(v) - 1
+            else
+                round(target_type, i_approx)
+            end
         end
     else
         idx_prev[]
diff --git a/test/runtests.jl b/test/runtests.jl
index 2d3a88b..4b95bd8 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -37,6 +37,7 @@ using SafeTestsets, Test
         guesser_prev = Guesser(v, Ref(1), false)
         @test guesser_linear.linear_lookup
         @test searchsortedfirstcorrelated(v, 4.0, guesser_linear) == 3
+        @test searchsortedfirstcorrelated(v, 1.4234326478e24, guesser_linear) == 5
         @test searchsortedlastcorrelated(v, 4.0, guesser_prev) == 2
         @test guesser_prev.idx_prev[] == 2