diff --git a/NEWS.md b/NEWS.md
index fdb9ca3..163682e 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -5,6 +5,7 @@
 * Updated the *in-situ* examples in the README file
 * Cleaned the `theme_bw()` from README and set default `ggplot` theme with `ggdist::ggdist()`
 * Fixed the error in README example for plotting the in-situ velocity-acceleration profile 
+* Fixed error in the `find_optimal_FV_scalar()` function and all upper-level calling functions which happens when `resistance` parameter is below zero
 
 # shorts 3.1.0
 
diff --git a/R/optimal-functions.R b/R/optimal-functions.R
index f804946..5e12976 100644
--- a/R/optimal-functions.R
+++ b/R/optimal-functions.R
@@ -80,8 +80,8 @@ find_optimal_FV_scalar <- function(distance, F0, V0, bodymass = 75, inertia = 0,
           par = 1,
           fn = opt_func,
           method = "Brent",
-          lower = 1 / min(100, F0 / resistance),
-          upper = min(100, F0 / resistance)
+          lower = 1 / min(100, F0 / ifelse(resistance < 0, 0, resistance)),
+          upper = min(100, F0 / ifelse(resistance < 0, 0, resistance))
         )
       },
       error = function(cond) {